guess -
visually inspect every part of each tire, otherwise, yeah, you could guess. Look. Listen for hissing. Feel for nails and stones.

Are the sidewalls or tread cracked from age and allowing some air to slowly escape?

The stem valve might be failing. Try those little black caps on the stem.

The bead might not have a good seal any more. Probably not unless the rim is really dented.

What the heck, that old truck is doing OK. Take out the bicycle pump and get some exercise each time before you go on the road. :-)

I thought about beads valve stems. And bicycle pumps. But the bicycle pump was in the back of the truck and somebody stole it. Thanks:-)

Best to go head and get four brand new tires installed. Check out firestone they are always running specials like 4 tires for under 100 bones, you could spend that much messing around trying to get the leaks fixed in the ones you have. I have seen older tires that actually leak thru the sidewall of the tire itself. I had to buy one for a manlift just the other day ($305.00 worth hopefully your is a bit more normal tire) it was leaking thru the sidewall of the tire itself and could not be repaired. The manlift is a situation were the tire sets alot outside and I guess they eventually dry rot a bit.

One souce of slow leaks (for tubeless tires) is the quality of the seal between the tire and the wheel. If you mount new tires or have an old tire re-mounted, make sure that the shop cleans the rims (where the tire contacts the wheel). Rust and dirt on the wheel is a source of slow leaks. One shop that I know says that 80% of leaks on newly mounted, or remounted tires is caused by dirty rims.

I'd also guess the tires are leaking around the rims. I've had this happen to me several times. They call it a bead leak. In getting mine repaired, I cannot remember if they did anything to clean the rims, but I do remember they used a rubber cement type stuff to get a better seal around the rim.

Rubber cement or no, the rim must be clean.
If the tire-changer man was as poorly trained as I was, I can imagine many leaks..
I think my leaks are now 99% over, thru the use of a sealant ( NOT "slime").

Another cause for air leaks is the man checking the pressure when warm and then bleeding some off to "correct".
The PSI figure is set at room temperature ,about 60 degrees, the warm pressure could hit 80 or even 90 degrees, but this I have never measured..
Roger Brown has written many pages on this tire and air pressure subject.