By the time you get to the Avenue of the Giants turnoff, you will be most of the way to your campsite south of Scotia. I would suggest just heading straight there and then returning in the tow vehicle to see the trees. Ancient Redwoods RV park has always had strong recommendations and is well situated. If you do the Founders Grove hike, take the short (really bumpy) road to the Rockefeller Grove and enjoy a less crowded area.
If you need fuel along the way, get it in Willits. I always fill at the Safeway on the way north. I can then reach Brookings, Oregon before I have to fill again. Fuel costs will increase as you head north on 101 until you reach Oregon.