First, I think it would be very cool to restore a 70s vintage Motorhome, but restore, not modernize from a cosmetic perspective. If it has ratty green shag carpet, find new green shag carpet. Orange and yellow flowers on the upholstery or curtains, find something similar in style. You get the idea.....
Watch the tv show Flippin' RVs, and do in a 70s style what they do to vintage travel trailers.
But, and this is the very big but........ You better have time, patience, money, patience, money, skills, money, skills, and patience. When all else fails, you need time and more money.
And after it is all done, patience when using it.
Watch Flippin RVs, and see what they run into restoring those old trailers, which luckily do not have old drive trains to deal with. Just like with old homes, every time you go and open something up to fix one thing, chances are you find 5 other things you have to deal with. And no telling what the drive train is really like after 40 years of basically sitting.
And things like the head, showers, and holding tanks are not nearly what they are like in newer coaches. Of course the electrical systems while simple, are primitive. The AC is probably marginal at best, and who knows what the furnace is like after sitting 40 years. The fridge is probably marginal at best, and fridges that use propane need to be very carefully examined by someone who really knows them (or just plan on replacing it anyway). It will be a lot more like real camping, than glamping in a new RV.
I love my cars, and love to use them. And many times I thought about getting some vintage collector car, but I always seemed to buy something new because I did not have the patience to own one that I was actually going to drive often, and not just sit in my garage.....
Btw, Tiffin may be able to still help you with some of the parts. There are also some independent shops in Red Bay AL that do all sorts of Tiffin work.......
Good luck, but go into this eyes and wallet wide open, not on a wing and a prayer. And remember, worst case is you get into it and find out it actually is a lost cause, so you may actually end up loosing it all, plus whatever you put into it.
2015 Tiffin Allegro 31SA, 24k chassis
2015 Jeep Cherokee Trailhawk