On the mechanical side, as mentioned, most share all of the major stuff; engine, transmission, AC Units, refrigerator, microwave, etc.
The house itself is where the differences are. Unless you're buying a true 1,000,000 mile bus (Prevost, Newell, Bluebird, etc) most are built using a lamination process. Basically it's all glued together. What is laminated can and at some point will delaminate.
I'm a big fan of hung wall construction. This method frames the house much like a stick built home. There are aluminum studs typically on 16" centers, headers over windows, etc.
Those building that way are Holiday Rambler/Monaco, Newmar, and Travel Supreme/Enterga. There may be another, I can't think off it now.
I would suggest to talk a walk around a few local campgrounds. Look at the older units. They all look great when they're new, see what ones withstand the test of time over 10, 15, 20 years.
Friends of ours have a 2002 Newmar Dutch Star, it still looks brand new. Other friends have a 2003 Travel Supreme. Same deal, looks new. Both of these have over 100,000 miles on them and they're still tight and right.
As always, your mileage may vary.
I just happened to think, ours is almost 10 years old new. We're camping now. Last night when we pulled in the people next to us asked if this was a brand new camper. Time sure does fly, seems like we just bought this one. :B