I'll merely echo the sage advice above: get what works for you and ignore anybody who's sure they know what you need better than you do.
There are advantages to both diesel and gas engines, with the overall cost advantage to diesel for people who put lots of miles on the rig. Folks who only put a couple thousand miles a year on the RV will never recoup the extra upfront cost. As a result, gas is better for some folks; diesel is better for others, and which you are is for YOU to decide.
In the interest of full disclosure, my wife and I bought a diesel car, a VW Jetta TDI in 2001 after decades of driving gas cars and trucks. Since then, we've only bought other diesels and love them. The two tow vehicles for our Travel Trailer have both been diesels, and have served us well. As much as we drive and how we drive, diesels suit us well, thank you. YMMV.