Dgsloat,
Since I dont know where you are, or where they are, this is a guess and assumptions... Im gonna assume it is COLD at night where they are at, and not needing the Air Conditioner, other wise it would been redily apparent that the 120v source might not be working? IE the campground plugin.
My guess is they are plugged into 120vAC power, but there is not power on the box they plugged into, shoud be asy fix.
And unless they use the 120vAC appliances, they might not have noticed such? Happens all the time at campgrounds I go to, the weak breaker fails under load... I went 1 whole day before I bothered to notice that my campspot breaker was blown before I even plugged in I think, because I didnt use the Air Conditioner or any appliances (micro, or TV), & because most everything runs off battery, Furnace, waterpump, LIGHTS etc. At least in mine.
So you need to find out, what IS working, (any appliances or AC) if not it is the source, if YES they do work, there is more trouble shooting to do.
They can probably hook up jumper-Cables to charge battery for 20 minutes with the TOW VEHICLE ENGINE RUNNING, then have enough juice to raise and lower top a few times. As you know, to let the roof down, got to lift it up off the safety ya know, then let it down. so if they are ready to just get home, have them shut EVERYTHING OFF, hook tow vehicle with jumpercables to camper battery and charge it for a little while. It could be bad battery, or could be your inverter has something wrong & not charging battery.
Lastly, If they have an 8way plug with factory wiring to charge battery, I ALMOST guarantee it will blow that fuse, for that power wire, if they dont get the battery charged up a little, IMHO.