First off, let me appologize to you for the delay in my response. I have been dealing with some very difficult virus and compatibility problems on my computer and every time I trie to make a post longer than about three paragraphs my browser would freeze or time out. I think I now have those problems corrected for the moment, but time will tell.

Some of the responses you got from plumbers are simply outright lies, although I can understand their resoning for having done so. Many tradesmen in all trades are reluctant to work on mobile homes because of the manner inwhich some mobile homes are built.
Whenever a structure is site built we have to work to the minimal standards as set forth by the codes which are in force at the site, but mobile homes and manufactured homes are not governed by your local codes.

The problem is that a mobile home is built many miles from the point where it will be set up and the manufacturer has no way of knowing which code would be in effect where that particular unit will be installed, so the jurisdictions allow an exception for mobile homes & manufactured housing.
If you look on the exterior wall of your mobile home about eye level near the front door or on the right front of the trailer when facing the tow bar you will see a small plastic sheild shaped medallion that says the motor home was built to meet or exceed all building codes in effect at the point of manufacture. The problem here is that most of the mobile home manufacturers are very careful to locate their production facilities in areas where there are no codes relative to manufactured homes, so they basically can cut corners any way they like and they are notorious for using sub standard materials & procedures. The problem here is that when we make a serious repair or rework that requires inspection it becomes a real challenge to convince the inspector what is our work and what is original.

PB pipe was first introduced in the late 70's and by the mid 80's it was touted as being the greatest thing since pockets on a tee shirt. In fact, nearly all the hype we are now hearing about PEX is a revamp of the old PB advertising. In fact, it was PB that introduced the manifold system for plumbing potable water distribution.
PB pipe is fine, but when they first introduced it they were using cast Polybutelene to make the fittings as well as the pipe. By about 1987 they started noticing a very high failure rate due failures of the compression connections on the fittings so they changed to the crimp ring type fittings that we use on PEX today. That seemed to solve the fitting problem but they began seeing an extremely high incidence of pinholes in the fittings from pipe wall erosion. In the late 80's a number of very big class action suites were filed against the PB manufacturers and it resulted in PB outlawed by the UPC (Uniform Plumbing Code) and there was a trust fund of about $2billion dollars to pay for the cost of replumbing any structure that had PB pipe. Unfortunately for you, that program has now closed. Now believe it or not, PB pipe is still listed as approved under the IRC (International Residential Code) although I don't know any plumber who would risk the liability of using it.
You are absolutely correct that rodents of all types, Rats, mice, rabbits and Opposums all seem to have a sweet tooth for PEX so with your rat problem that would not be a good choice for you either.

This then leaves us with Galvanized Iron Pipe (GIP), Brass Pipe (BP), Copper pipe & CPVC pipe. We can instantly rule out GIP & BP as cost prohibitive and labor intensive to installl, not to mention that GIP has a relatively short service life when compared to the alternatives.

So we are now down to copper & CPVC. Now if you lived outside the municipal water supply district my choice, hands down, would be CPVC.

Why you ask? Although most of the ppl who frequent this board instantly badmouth copper at the mere mention of acidic water saying it will cause pinholes, that is not true. Tests have confirmed that there are 5 leading causes of pinholes in copper and acidic water is #5 on the list. The first 4 leading causes all relate to improper installation of the copper, yet I doubt if anyone reading this board knows what they are. In fact, it has been detertmined that even with a high acidic rate, there are a couple other chemicals that must be present in the water to cause pinholes in copper and those chemicals are generally only found in water that is sourced from super deep wells. In fact, the highest incidence of pinholes in copper from acidic water occur in arrid regions where the wells are often 400' or 500' deep. In the United States the highest incidence of pinholes caused by acidic water occurs in New Mexico, Arizona, S. California and Utah.

Now as I stated before, if you lived outside the municipal water district I would definitely specify CPVC, but since you are on the municipal water supply you are okay because there is a Federal Regulation that says all municipal water suppliers must treat acidic water so it will not harm copper pipe.

So now to answer your question, should you use copper or CPVC? Obvioulsy my first choice would be copper, but you expressed that copper would be an extreme financial hardship at this time.

Might I suggest a hybrid system?

For a mobile home you have basically a main & branch system where you have one long main line running end to end with branches off to the bathrooms, laundry & kitchen. I would suggest you use CPVC to make the main and branches with a copper riser from the branches up to the fixtures. You could make a small opening in the wall at the fixture, then pre-assemble the copper riser with the end use fittings and valves. That line could easily be slipped into the wall and run down through the floor into the open crawlspace under the mobile home. In this manner all the soldering could actually be done outdoors thus negating your fear of fire.

Once in the crawl space they could use CPVC to make the branch connections and the main runs thus saving you considerable money, and it would be easy to come back at a future date and upgrade that to copper if you like.

Now the plumbers claim that he doesn't know how to run CPVC is telling you an outright lie. CPVC and copper are both installed identically the same except you glue CPVC and solder copper.

And lastly, your question concerning "Biofilm"
Under laboratory conditions where they take a foot long section of new pipe and attach a cap to one end, then stand them upright in a rack and fill the pipes with tap water Biofilm will develope the quickest in the copper pipe, followed very quickly by PEX, then a short term later in PVC and finally after a fairly long time, CPVC however; under real world conditions as water flows through copper it very quickly forms a dark green or black microscopic thin layer of film on the inner pipe wall. That layer of film is copper-sulfate, which just happens to be one of the best anti-bacterial agents known to man. Thus in the real world by the time Bio-film has had a chance to start developing the copper has already developed a counter agent to kill it.

On the other hand, the Plastic Pipe & Fittings Associations installation handbook says that all plastic pipe should be sanitized at least once per year unless you are on a municipal water supply that chlorinates the water.

The PEX installation handbook says PEX should be sanitized twice a year.