Hi Tony.
If this problem doesn't present it's self when you're out on a site hook up and using the same fan heater etc, I would suspect the garage supply. If you can, plug the van into the eleciric cooker 13amp socket in the kitchen. This 13 amp standard plug/socket is usually the only socket wired into the cooker circuit. If you can't do that, use a socket that is not on the same trip/fuse circuit as the garage.
If your consumer unit (fuse/trip box) is not marked up, just plug a table lamp into the socket in the garage and turn off the trips individually to find the one feeding the garage from the house.
Now try the heaters out and see if they trip.
What you might be experiencing is a back feed via the neutral circuit in the garage wiring. This is quite common and as the voltage is usually below 20 volts, you don't see any problems. But the circuits of caravans are wired via the charger unit which does detect neutral back feed and shuts the van down via the trip.
From a safety point of view, unless you are an electrician, please don't risk playing with the wires as electricity is the silent killer. Get aqualified electrician to do the work.Even sparkies like me can get caught out.
I was decorating our front room of our newly purchased home. Got to one of the enconomy 7 storage heater points, and remembering that I'd turned that consumer unit off at the isolator circuit, I stupidly presumed that it was a dead socket. Un be known to me, the previous owner had wired a night storage socket to the 13amp household plug circuit (which was still live), I took the cover off slacked the terminals and grabbed the wires to stuff them into the box out of the way. I was blown across the room by the 240 volt shock and was very lucky to be alive to tell the tale.
Hopefully you'll be able to narrow down the problem and get a sparky to fix that issue.
Good luck.
Steve L.