Going West, need info

[wat42]

Heading to Yellowstone from Ohio area after July 4th, will be traveling I90, not I80. Due to medical reasons, wasn't sure we could make trip. Do not have camping reservations. Any suggestions for places to stop along the way, campgrounds or is there any free overnight camping allowed on I90 or places off I90. Would like to stay a week at campground near Yellowstone, not necessarily in Yellowstone, don't mind driving back and forth if you have suggestions. We do have a generator, any free camping spots along the way? Have not been able to get plans made like I wanted. Your input is appreciated. Taking a small 21' travel trailer and F250 Diesel truck, any road problems I should be aware of. Will be going on trip for 8-12 weeks, going to some of the other national parks, Cody, Salt Lake City, Grand Canyon, Texas and then heading back on Southern route. Do not have to hurry so any info will be appreciated. Also, anyone stay at Wal-Marts or the truck stops? Where do you get water if you don't stay at campgrounds? Thank you.