I wish that park reviews would ask reviewers to include the size of their RVs. A motorhome camper with a 32 foot rig will not have usable information for a rig 40 feet or more, but the size of the motorhome/trailer/5W is usally not mentioned in reviews. I'm sure that the same information would be helpful for TTs and 5Ws as well.
Start with this info website. Best time to miss the big crowds are May to early June, and late September. The website will give you a better idea of when to visit according to what you would like to see. For example, wildlife babies, or the wildflowers in bloom, or the ungulates in rut. The Tetons are always less crowded than YNP. You should have little problem finding a CG for your truck. Be aware that the parks require hours of driving in order to visit all the sites. When going through YNP, stop by ranger stations before setting out. They will know of any animal sightings, road closures, and have suggestions on how to best spend your time allotted for their area of the park. Get your days started early in order to see the most wildlife and miss the most traffic. When treking away from the boardwalks, try to hike in a group or at least have bear spray for each person on the hike. The bears have already left their dens this season. I would advise leaving all pets at home if that's a factor. None of the National Parks are pet friendly. Hope this helps.