I don't want to try to tell anyone which is best, gas or diesel, because I have never been a professional driver nor a professional maintenance person. I have driven my '99 Allegro Bay on a '98 Chevrolet chassis for more than 91,000 miles with no major chassis expense. I get an oil change every 5,000 miles at any local Chevy Dealer, usually for a cost of about $39.50. I took my RV to a Motor Home Service establishment one time and they charged me $97.00 for an oil change. I have driven my Chevy gas powered motor home over every mountain range in the eastern half of the USA, including the Rocky Mountains. We did not go up hill as fast as some diesels, maybe 35 MPH, but we got up one side and then down the other.
By the way, I have only one slide-out, in the living room area, and will never have more than one. If I ever buy another motor home, it will have to be special built with one slide out. In my opinion, two or more slide-outs, the second bathroom, the washer and dryer, the dishwasher, three or four TV sets, the sitting area in the bedroom, the double wide refrigerator, with icemaker, and all the other high-end luxuries, like Italian stone or fancy wood flooring, mirrors on the ceiling, lights mounted every place the manufacturers can find to mount one have ruined the motor home business for the average motor homer who likes to get away from home for awhile.
It seems that most people in the motor home world wants to take home with them when they go! All these fancy foo-foo items do is to use up weight that could be used to carry important items for every day living and cause more upkeep and expense for repairs in the future.
When I first bought my 36 foot Allegro Bay I had it weighed. It is rated for 26,000 pounds braking weight. Completely loaded with full water, propane and full gas tank, and with the Honda, the total highway weight, was 25,450. I had it weighed every year, for about 10 years and the total weight was always the same plus or minus a couple hundred pounds. At 65 MPH, I got 4.2 MPG. When I started driving 55 MPH, I got, and still get, 6.8 MPG, an increase of about 50%, and I add one quart of oil every 1,500 miles or so. Can you imagine driving 55 MPH for 91,000 miles? I did it and my wife and I enjoyed it! I don't know if a diesel would have been better or not...I just don't care...what I have is sufficient for me and my desire to enjoy life, not impress other people with what I drive!
When I was driving 65 MPH, I drove 6 to 10 hours a day almost every day and when we got to our overnight destination, my wife and I both felt, worn out, shook up and frazzled. At 55 MPH, we also started driving less per day, more like 4 to 6 hours, some days even less, like 80 to 180 miles, and we arrived relaxed and ready to enjoy something in the local area, like a movie, sightseeing or eating out. One of the main arguments for 55 MPH is that you always have an open lane in front of you, even in high-traffic conditions!
Say what you will, some people like gas and some people like diesels. No one in either group should belittle the other nor should they try to convince anyone of which is better.