If you are talking about a 1970's vintage a big slow turning heavy mechanical diesel will outlive a carborated gas engine with less ongoing maintenance.
With modern engines, there is very little difference in engine lifespan and gas engines require very little ongoing maintenance. You might be looking at an oil change per year (at less than half the cost of the diesel oil change) plugs at 100-150k (honestly haven't changed plugs or wires in a gas engine in years. Probably should but just not an issue). With modern electronically controlled diesels, they are at least as complicated if not more so.
As long as the gas engine is properly sized for the load, no reason not to expect 200-250k miles of reliable use. By the time, you hit that point the house is likely 20-30 years old and has reached the end of it's life.
If you do have to do major work or replace the engine, the gas is going to be drastically cheaper. You may want to shop around for a non-RV shop but with a big diesel, you are limited to RV or truck repair places.
So if both drivetrains appear to be in good shape, I would choose based on the overall condition for similar rigs.