In my opinion the answer is: it depends.
You need to evaluate your own ability to fix things, both on the chassis and the coach "stuff". If you are handy on mechanical things you are far ahead in the game.
How many previous owners? How was the B stored (covered, indoors or outside)? What part of the country did it call home (salt in some areas, extreme heat or cold, etc.)? Was it used a lot or just stored a lot? Generators, for only one example, can clog up without use once a month or so. And they can cost a bundle to fix.
If you are new to RVing, I would suggest you find someone with experience in motorhome ownership to help you evaluate any prospective purchase. Age alone is not as great a factor in my view as other things. You mentioned that what you are seeing is Ford, Chevy and Dodge gassers and that is exactly correct. Each have their own idiosyncrasies in some periods or years. I have had all three and find them all acceptable. The Dodge 5.1L (318 cid) engine was used a lot in those years, for instance. It is great little motor. Chevy 6.L have been around a long time and, like the Dodge, are really stout and proven.
All the rv "stuff" can be replaced but it can sure get expensive. In the long run, you may be out fewer dollars in total on a 12-15 year old rig.