The 5.9 is based on the 12 Valve which has been around since 1983 I believe. It was designed to operate at 100% duty cycle all-day long in tractors and pumps. In trucks and highway vehicles you should get no less than 500,000 miles before major overhaul assuming all regular maintenance is done and the cooling system is maintained.
Your concern more should be aimed at the transmission and all the systems and seals of the motorhome. The engine, while underpowered for your application is good.