Come to think of it, I even stayed one or two night of stealth camping in Boise. I tore up a tire in the Alvord and went to the Boise Sears for a replacement. They ordered a D instead of and E load tire. It was already late in the afternoon when I found out. I stayed overnight and I believe even the next night in the Sears parking lot. It was plenty safe with no reason to sleep with one eye open. I did not see any signs but I have no idea if staying was within local ordinances. I stealth camped and looked like another vehicle waiting overnight for Sears service.
I don't know what you mean by "RV spots". I did not see any in the Sears parking lot or in the parts of Boise I drove through.
Nor would it have mattered if I was in a Class C or Class A or towing a trailer. I needed a place to stay.