Vents can get obstructed over time. First, I'd wonder about what filters you are using. If they are diminishing airflow, then that might be your answer. Then I'd try to visually inspect the ducts - perhaps there is a flex duct that got kinked. Also a significant leak could have developed and thus lowering airflow to the distant areas.

So filter check first then inspect ducts.

I was just told by an installer to run your fan 24/7 and it will stabilize the whole house. He told me to picture it like a bottle of italian dressing if you don't circulate the air it will seperate and you will have hot and cold zones.

rbon, if you have average to high humidity in your location, this practice could increase the humidity in your home to uncomfortable and dangerous levels. It will increase your electricity consumption as well. Bad advice, bad installer.

rbon,I don't think that the fan will stabilize all the things,definitely it will increase the electricity consumption.I am amazed to see that in what context does the installer said this because its not the appropriate solution.

Are the registers in the ceiling or in the floor? Double wides that have floor registers have a 12" or 14" crossover flexduct under the trailer that sometimes comes unhooked. I'd check to make sure you don't have any air loss from there 1st.