Our TT is 35 from hitch to bumper and overall length is 55 feet.
I have no trouble getting in and out of any spaces or getting fuel. I've even been forced (by an empty tank) into a few tiny stations you would not think are good for any sized trailer. In one such tight spot I actually drew attention of a couple of on-lookers who cheered for me when I squeezed by all the other vehicles and obstacles without having to stop and back up and didn't trade paint with anything.
We have been seriously considering trading up and all the features we'd like to add equal out to a few more feet in length. I'm fine with 37 - 40 foot range. I haven't had any trouble finding campsites long enough, either.
I have found the site size issue to be a little strange. I've been places where the staff said I might not fit and found there's plenty of room, only a tree on one side of the site makes it hard to back into (and I assume that's why they think a 35 footer might not fit). Other places will let you get away with hanging a few feet over the back end of the prepared site as long as the wheels are on the pad. Only been to one State Park where the staff said all of the TT (stabilizer jacks and all) had to be on the gravel pad and that just meant unhook and park the tow vehicle beside the TT, no big deal.
Drive your rig around and get used to it and then take your time and be careful. You'll be fine.