Backup camera recommendations

I'm considering getting a backup camera system. I'd like one I could have on the back of the truck, then move to the back of the camper. My truck doesn't have a fancy radio with screen on it, but I do have a mount for my cell phone, so something I could view through that would be good. I saw an ad for one for $25,but thought that might be an El cheapo.
I will be renting or borrowing a camper to start so I can't permanently mount anything. Thoughts?