Short answer:
Get as long of a spot as you can...IF you can. You are a HUGE rig when considering Yosemite Valley camping.
Long answer:
We camp in Yosemite a few times a year when the booking gods are on our side, lol. Just getting ANY spot is your first concern. Be aware that each site has its own length restriction that may or may not include possible over hang at the back of your trailer. The issue in the valley at say Upper Pines CG is that while the trailer may fit on the pad/spot, you may not be able to maneuver around the very small CG roads in some of the loops. Additionally, you may or may not have room for your TV anywhere near your campground spot depending on the site, trees that may or may not be in the way, and who is next to you. You will possibly have to park at Curry Village about a half mile away. I refuse to use the new name or can't remember it ;). Even that may be difficult at the end of May.
Our last spot at Upper Pines a few weeks ago during Thanksgiving didn't allow room for our slide out to fully extend or for our large step to be fully extended thanks to trees on each side. We also couldn't hang over the back of the spot because the fire pit was in the way. We were only 24.5 feet total in a class C with no toad. This was an extreme scenario at Upper Pines and the worst it has ever been but that was the only spot left to take and there it is. Meanwhile, the guy next to us had a 40 ish foot class A and had all the room in the world with his 3 slides extended and wedged a toad in as well. Sometimes the site descriptions are great and sometimes not. The camp hosts and rangers are very nice but they are a bit more militant than most places what with all of the people coming and going.
Again, the larger the RV is, the harder it will be to get a spot INSIDE the valley DURING PEAK CAMPING TIMES. In fact, almost impossible with a rig above 25 feet. Millions of people a year visit from all over the world.
I recommend staying at Indian Flats CG just outside of the park on 140 in El Portal if you can't get a valley spot. It's nothing fancy but it has full hooks, almost always available, pretty cheap, VERY close to the valley, and a nice hotel next door with a heated pool, hot tub, and a restaurant free to use as the same people own both or something like that. There are other options but none closer. Additionally, check the road length restrictions heading into the valley. I'm not sure what your TOTAL length is or what the various restrictions are.
Good luck! It's a bit of a pain during peak times and you are going during peak times, but well worth it to see one of the most amazing places on this planet.
