Can you camp in the park and if so is it set up for RV trailers and is there a maximum length they can be?

There is a 22 foot vehicle length limit on Highway 198 (south entrance). I guess that includes trailer length, but, I'm not sure. Google "sequoia road restrictions" for current info.

Yes there are sites that can be reserved and some that can be same day first come first served. You can find the maximum RV lengths at the reservation page since it varies per campground. Check recreation.gov

Yes. There are designated areas for camping.