There are probably a lot of things that should be checked, but I'd start with all parts of the axles and suspension, brakes, wheel bearings and seals, the hitch, tires (if the date code is more than 5 years old, replace even if they still look good), any roof-mounted components (particularly the A/C mount bolts and foam seal), and the caulking around joints and seams. These are safety and water damage items, and then start looking at the less important and cosmetic things. I'm sure I've forgotten about some other essentials, but others can chime in on those. That should keep you busy for a while. Don't know about the mileage - seems to me if everything either checks out or gets repaired, you're good to go indefinitely - for as long as you still like the unit. JMHO
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.