Thats a gasoline budget even at 60K. New Class C with sleeping room above the cab is possible. The maintenance on an old diesel pusher is substantial, you gotta be ready to rebuild a 10K plus transmission or loose a 15K plus engine, radiators, intercooler, turbo and just the fuel injectors can cost thousands. The little 275 to 350 hp diesels work hard in those old pushers, condition is very important. The older rigs could be more costly to maintain than a late model gas motorhome.
I prefer 5th wheels or trailers so if the truck dies for whatever reason I can quickly find a replacement or even borrow one on short notice. On the road I can put the truck in a shop for repairs while I have the 5th wheel set up in a good spot and enjoy my time there.
