Diesels aren't fully run-in until they've done 50K. When I was a transit driver, some of our coaches had over 400,000 miles on the clock and were still just fine. They were almost all Gilligs with 6-cylinder Cummins motors. A few of the 40-footers had V-8 engines.
AFAIC, mileage is not an issue for the engine and transmission - damned things last for ever if they're properly cared for. A bigger issue for me would be the RV's unique systems, furniture. carpets, etc., but that would be based on hours of use rather than age.
Caveat - I was never in the DP market-place due to price, though we would've bought a 30'DP (a Charleston) if the dealer had been more friendly.
