Yep, at least the Flying-Js are. Many have special RV areas for parking overnight. I use the one in Brunswick, GA as halfway stop between home and FL. I get 3 cents/gallon off with a loyalty card to boot.

Also, many Cracker Barrels have an RV spot for staying overnight. Also, most but not all Walmarts let you park overnight. Local ordinances sometimes prohibit that. So, you need to ask "the desk" to avoid a cop's knock on the door in the middle of the night.

Out West many casinos have free overnight parking or at least below market rate camping.