The maximum trailer length permitted on the road to the Basin is 20 feet, so says the sign at the beginning of the road that takes one through Panther Pass down into the Basin.

I was there in 2017 (my pre-Campendium days) and managed to drive my Rialto (22' VW class B) up the mountain. Best advice - pay close attention to the reservation details, site length, equipment, etc. I started off at a site that was in no way set up for an RV, but thanks to the host, was able to move to a better site. It is an old school campground but SOOO worth a visit. Allow plenty of time to explore.

It's very tricky. I pulled my 16' Jayco with a F150 crew cab up that road. You can make it, but not easy.

There are a limited number of trailer sites. Last time I was there people had to move vehicles when I parked my 16'.