Welcome to the forum!
I run into the same problem with my 76 Argosy MH. Normally if you tell them it is a Chevy, P30 Motorhome chassis they can find it. I sometimes do not tell them it is a Motorhome until they find the Chevrolet P30 section. Some of the computerized parts lists have just P30, othres break it down with Heavy Duty (motorhome) and just P30.
Most of the big retail chain stores like Advanced, AutoZone, Checkers, can help, but more often than you would like you get the Waaaaaaaht??
Napa seems to be better and I can even tell them it is an Argosy Motorhome and they find me the parts.
Now for the gotchas. The parts books are often wrong, or a PO or PO's Mechanic has at some time replaced parts and used the wrong ones. My U Joints were a perfect example. The front one that was in the book at NAPA was right on. The rest of the ones they sold me were the wrong size. When I returned them they said, "we can get you those, but that is not what the book says the manufacturer installed". I doubt tha someone upgraded the U joints on my coach
If possible it is best to try to take the part with you, that is why I am heading to NAPA today to return a carrier bearing that was the wrong type.