I used one in my pickup camper last fall, and nearly froze to death! Well, not really but I was cold enough all night that I got very little sleep! They seem to just stay cold where a mattress warms up from your body heat.
We used to use one when we tent camped, and yes, it kept going bad, but the one we bought had a lifetime warrantee, so I just kept taking it back every couple times we used it.