There's only a few ZILLION opinions on what's the best potential motorhome out there in RV land. Everyone's got their ideas of what they think and or use, as a good/great unit. Gas/diesel/class A/C and all that. A general thought might be, look for a higher end coach, that's a few to quite a few years old. That way, you get a nice, higher end coach, and don't have to pay an arm and a leg for it.
If you lean towards diesel powered coaches, there's not a lot of engine choices. CAT pulled out of the races years ago, around '08 or '09, can't remember the exact last year of CAT powered coaches. That leaves Cummins from that point on, for supplying just about 99.9% of the diesel powered coaches with the exception of some fairly high end units powered by Detroits.
The CAT powered units are good units. CAT just decided to throw in the towel way back then due to not wanting to put with all the smog stuff for over the road engines. They're still kicking bu... in the construction/off road industry.
As for gas engines, well, just like CAT and Cummins, GM yanked itself out of the RV market years and years ago which, for the most part, as in about 99.9999% of the gas RV industry, FORD is the main supplier of engines. And that primarily is the infamous Triton V-10. It's a good engine. But, based on it's design, it likes high rpms. The potential consequences of that kind of operation lends itself to NOISY front cabin areas, especially when ascending grades etc. It's a semi-common complaint from many Class A gas coaches.
There are one or two coach builders that have developed sort of a bad rep for dependablity and longevity. One that comes to mind is THOR. Now, before any THOR owners get their panties in a wad, I'm not saying ALL, one more time, I'm not say ALL THOR's are bad. It's just that there have been plenty of complaints on here and other RV forums about work quality, long periods in the shops, poor dependability. We have friends who have a brand new, 2020 Thor Windsport. And that coach has about 1,500 miles on it's now been in the shop for over 1.5 months for multiple electrical issues, main viewing screen on the dash replacement, computer controls for coach functions needing replacement, outlets that don't work and a few other odds and ends.
And those folks aren't the only ones with THOR issues that we know.
Now, we have an '04 Itasca Horizon 36GD with the CAT C-7 330HP and the Allision MH3000 6-spd trans. That coach is now 16 years old, has 94,000 miles on it and, it's been a great coach, minus some repairs here and there due to standard wear and tear. That C-7 has been a great engine, powering us all over the U.S. more than a few times. Anyway, good luck on your search and whatever you come up with.