Booked Loop B site 36 - on-line said it wouldn't take a trailer larger than 12'. 1st - it isn't a 'back-in site. You pull in along the roadside bounded by boulders for your site. I pulled in with my 19' trailer, towed by a large SUV without a problem.
We were here six nights and never had more than one other trailer in the entire loop.
Yes - right now you have to be fully self contained because the rest rooms are under renovation. That was no problem for us.
Although I haven't seen reviews specify a dump site, Wawona has one. It's on the way to town - left turn before the bridge and head to the end of the short road hidden on the left.
Much to our surprise my Verizon hot spot worked fine - even for streaming.
Oh - the Rangers were MORE than helpful.
If this is your first time in Yosemite (as it was ours) the speed limit of 35mph is real. Pulling a rig any faster on some of this curves would be crazy - and the number one cause of bear deaths in the park is the automobile......just sayin