We don't camp when it's all that cold out, but the roof heater has been more than enough for our needs when it is chilly (50's). So much so, that I've taken to no longer carrying a portable heater when we go out. Why take up the space and weight (however small!) if the roof heater is always there, already there, and works.
Doubt I'd use it for colder weather camping, but for our current needs it suffices. If we run it at night, I don't put it on max heat, it gets too warm.
Now, we have the low profile Carrier that is no longer produced, and I also got the upgraded heat strip from Carrier (under warrantee, there was a recall on the original strip for fire hazard). The upgrade, it seems, was of a lower wattage than the original, but it is still sufficient for us.
Since it clearly depends on the model you have, I'd try it and see -- with a backup plan, naturally, until you are sure. I happened to have several small portable heaters we no longer use in the house, so taking one or leaving one was simple, I didn't go out and buy something just to toss it in the closet. I've done that for a lot of other things
But not the heater...