It will last as long as you're willing to put in some basic maintenance and are willing to look at it.
The biggest factor that influences people to get out of an RV is:
1. Buy another one
2. Stop RVing
Tons of stuff has changed in the last 20 years in the RV world. Wasn't that long ago people were in aww of slides. Now units can have 4 or more of them. Floor plans have changed tremendously and that is a huge reason why people don't keep them.
However if you can avoid the lure of the new and better you can keep your old unit as long as you are willing to keep the seams sealed and roof intact. If you can lay a bead of caulk or turn a wrench you can fix almost anything in an RV yourself. Bit of youtube, tool and some desire. These units are not that complicated, especially the towables.