Some people are Chev fans and other Ford. Is one better than the other..that depends which fan you are talking to.
Best thing to do would be to have a mechanical check done on older units. That way you can know what condition it is in and also you will know what you have to fix if you buy it.
Once it is out of warranty the best engine is the one that has had the best general maintanance. Unless you get one of the few with records you can have a mechanic make some tests or take your chances. Modern engines will take a lot of abuse so your risk is low in my opinion.
2003 Newmar Mountain Aire, Workhorse W22, 2008 Saturn Vue, Falcon 5250, & US Gear Unified Tow Brake
Definitely get a separate mechanic to check over the engine, transmission, etc. If you know someone who has an RV and knows about the systems, see if they will check it out for you.
We have a 2002 35' Winnebago that we paid $29k for. We did not have someone else check it and wish we had. Don't forget to check the recall lists and ask if the RV has had it done.
In deciding on whether to buy a gas or diesel, I also checked about maintenance since I don't change the oil in my car and we are retired. Check to see what the cost of changing the oil and filter for gas or disel in your area.