Engines and transmissions will last as long as they are serviced properly. Motorhomes so often get neglected. I'm anal about my 1990 model and drive it every two weeks all year and exercise the generator. I have the P30 Chassis, class A, 55,000 miles-ish, 454 V8 engine, turbo 400 transmission. When we travel I drive it easy. I'm in no drag race and I am pulling a Honda CRV behind me. I would disagree that the engine and transmission are on borrowed time. Neglect is the enemy.
I have had to fix roofs leaks from time to time. Rubber roofs are just that way. I hate a rubber roof but, that's the most common and they never ask me before they designed it. If you get to the leak as soon as you see any sign of trouble then you can head off major problems. Ignore it and that motorhome will be ruined in a very short period of time.
If I were in the market for a used motorhome, and I kinda am, I would look by priority. The basics are no leaks that haven't immediately been fixed, no mold period. If the unit has old tires the price better account for me having to spend lots of cash for a new set of tires. Good servicing history on the chassis. The generator needs to test out good. They tend to get neglected bad. I would be less worried about things like refrigerators, furnaces, etc.. They can be replaced if the price is right. Carpet can be replaced.
Check the master cylinder, feel the brakes, look for leaks. Stopping is important. I wouldn't worry as much about brands on used motorhomes. Winniebago does make a good motorhome but, there are plenty of others too. The chassis is always the same, Chevy, Dodge or Ford. The accessories are all the same Dometic, DuoTherm, Coleman, etc..
There's a lot of orphans out there (motorhome brands that no longer exist). I own an orphan: Mallard. Means jack squat to me.
Pay attention to your floor plan. In our class A the sleeping is limited. I'm looking for a class C with an overhead bunk and preferably a slide out. We have a lot of "basment" storage and big water tanks. I would like the same in a class C, if possible. We have a spare tire. Lots of modern motorhomes don't have a spare. These kinds of things are important down the road.
