Welcome to the forum!
Have you rented an RV before? Make sure you both know that you like the RV lifestyle before investing alot of money or going into alot of debt.
I know nothing about diesels, but have read that they last longer...probably longer than I will be RVing, so I don't care about that so-called benefit. Also, just because they last longer than gas engines, does that mean they have any less problems over time? What I do care about is that I've read maintenance is more expensive on diesels.
We've had our two class A gas motorhomes up and down many of the mountain passes in all the western States with no problems. Sure, it's slow going up, but we're in no rush.
I would never finance a depreciating asset for 20 years. You could easily end up being upside down (owing more than the RV is worth). Life happens. You may end up having to sell it before you would have expected, due to illness, for example.
Lots to think about. Enjoy the process!
2002 Itasca Suncruiser 32V
2012 Suzuki Grand Vitara