A lot of very optimistic opinions so far. I have a 97 5'ver that I leave down south now. It is one of the oldest in a park of over 600 sites.
Prior to that I had a '96 MH which we would go cross country in. I saw very few gas MH that were older then mine with out of state plates on them. There would be a few more old Diesel MH's, but still not an abundance of them.
Remember as a traveling FT you will be driving your home down the road. Just think what that action would do to a S&B house. When you buy your rig make sure it will have as little twist as possible. The rig needs to be rigid so seams and junctions do not start to open up. As others have stated, once water gets inside then you got problems.
IMO, you get about 10 years of good solid usage out of a RV. During the 10-20 span you'll be fixing things to keep you busy. After 20 years if you're really using the rig, then it is all a bonus and it could end as soon as a repair pops up that costs more then the rig does.