Yep, you're too long for the road into Sequoia NP from Three Rivers. But I highly recommend going to King's Canyon NP instead. It has two beautiful groves of sequoias too, including the General Grant. And campgrounds are much easier to find and to get into. We loved the dry camping (water and a pay dump available at the cg) at the USFS Princess CG, located east of Grant Village between the two halves of KCNP. We had paved cg roads, a whole loop to ourselves (on a weekend!), and a site that was completely shaded by sequoias. Gorgeous!
If you really want to go to Sequoia NP too, you can go there directly on a park road from Grant Village in KCNP. If you still want to go to Three Rivers, Three Rivers Hideaway RV Park sucks! Too many permanent residents--I counted 13 sites with very old, rundown RVs--and full-time residents who like to leave their vehicles parked half on the access roads and won't move them for the short-timers. Too many trees too close to the road to back a short 5th wheel into some spaces, manager gone most of the time, one washer and dryer, and an uphill road to the dump if you don't get a FHU site (which we reserved but didn't get). Probably the worst RV park we have ever been in and that's saying a lot because there are a lot of pretty bad ones in Alaska.
"Retirement is the best job I ever had!"
2015 RAM 2500 4x4 crewcab 6.7L CTD; 2016 Rockwood Signature UltraLite 5th wheel