We tow our Scamp
13 with a FWD Pilot. There have been a couple of occasions I have wished for AWD due to exactly what you describe: wheel slippage going uphill on a loose surface.
One local lake is three miles off the pavement, mostly downhill with one steep section. No problem getting in, but that one slope nearly defeated me coming out. Electronic stability control doesn't like wheel spin, so it shut down the throttle and braked the whole rig to a standstill halfway up. Had to back down, turn off ESC, and take another run.
I'd say if it's just campgrounds, FWD will be fine. If you plan much backroad travel on dirt or gravel, I'd hold out for AWD. And yes, with a used Highlander, you'll want to find one one with the factory tow package for the ungraded engine and transmission cooling and heavy-duty alternator. You may find that rules out a number of the FWD units.
BTW, most AWD systems operate as FWD under normal conditions. Even Subaru's system sends 90% of torque to the front wheels under normal conditions. But when slippage is detected, it can send up to 90% of torque to the rear. I owned a Subie for 14 years and it's a great system. My wife's current '06 CRV can only do up to a 50-50 torque split. The one Honda offers in the Pilot has a locking mode for low speed use in slippery conditions, which would have made a drama-free ascent on that lake access road. In short, AWD systems vary. You need to research the one you're looking at.