Best route? “Best” is a little subjective.
I’ve done that drive a few times, but normally from Lone Pine (or nearby Ridgecrest) to Vegas; in opposite direction. Because I like a challenge, I’ve gone through Death Valley on 190. I also wanted to follow part of the Furnace Creek 508 bike race route to drive the climbs and descends.
Climbs are some of the toughest I’ve driven, and heat can be an issue in summer if in a hurry and putting your foot to the floor. In a small 4-cylinder pickup or my V10 van it wasn’t an issue at all, but on a loaded down rented 30-foot E-Series Class C (no toad) with almost 100,000 miles, I had to take my time on steepest portions. There were a few sections where around 25 MPH was all I wanted to risk for fear of overheating in late May or early June — can’t recall exactly.
I don’t know if there’s an RV length limit, but since there is camping in NP it may be OK. We only drove through, but I’d like to overnight next time we are there — if there is a next time.
I’ll add that I enjoyed driving through the National Park and appreciated the barren landscape, but my wife not so much. I think it made her a bit nervous in part because rented motorhome wasn’t in great condition. Also, gas was the most expensive I’ve ever seen, but when touring I consider that normal part of cost.
For what it’s worth, 395 is scenic and worth driving in my opinion. I even got to ride my bike on parts of it while visiting my daughter. There’s a little campground at end of the Mt Whitney Portal Road I also want to stay in one day. I only rode up there on a bicycle, but recall the campground is tight, so a larger motorhome may not be able to maneuver easily if at all.