We have a 40-foot Class A and, in retrospect, I wish we'd gotten a smaller rig, mostly because we enjoy more remote camping. The smaller you go, the more "exotic" places you can reach. The larger your rig the more limited you will be by roads, trees, mud etc. That's not to say you can't find boondocking spots with a larger rig (we do), but you will be more limited. I have to think about our size (and weight) whenever we go boondocking & I can't risk driving somewhere the rig might get stuck.
The SW desert in winter is not much of a problem (lots & lots of space out there, with many firm roads), but forests can be much more limiting. Moisture will make roads softer (big issue for heavy rigs) plus trees, turn-around space and overhang will affect how far you can go. With bigger 5th wheels you also need to think about the overhang/clearance at the back. We went boondocking in the Eastern Sierras with some friends last fall who had a large DRV...gorgeous rig, but their overhang (at the back) was so low & bad they kept scraping the road on their way out to the site. We ended up having to boondock much closer to the main road than I would have liked. The lighter & smaller rigs have better clearance.
So, just some things to think about.