Winnebago's construction is somewhat better than other volume-produced Class C's. Much more so if WBO has a fiberglass roof and the comparables do not. FG front cap is a plus, I wouldn't call it a deal breaker. Deal breaker is both Flat and Not Fiberglass. Some WBO have what I call a "Tiara." Just a little "mini cap" at the top of an otherwise ordinary "nose" made of siding material. What the Tiara does, it allows the front edge of the roof to be crowned like the rear. It also mounts the Clearance Lights on a surface that could be easier to keep sealed, than siding. It's also better to NOT have a front window in the cab-over.
Many of us have bought prior rentals. Many are very high mileage, entry level coaches, and a number of buyers seem to be very happy with them. An outfit up in Anchorage (Great Alaskan Holidays) sells off its WBO Chalet Class C's (a brand produced for the rental market) with lower miles than most other rentals and they are simply beautiful. Easily confused with new.
WBO excels in owner support, with their availability of service and parts information, and of parts themselves. They are wired more like automobiles than like other RVs, which are wired more like houses. Most RV's you know this switch is supposed to work that light, which is OK till it doesn't. WBO can sell you a new wiring harness and show you where it goes.
OH, 20,000 miles is no big deal. None at all. You probably don't want 100,000+, but I'd rather buy a 5-YOA coach with 60,000 maintained miles than 5,000 ignored. Coaches don't stand around well.