Eddie, as you state that testing is not your department, it makes perfect sense to get a suitably qualified engineer to check it out.
In the meantime a simple check you can do for the electric side is put the fridge on mains for about half an hour, switch off the mains, remove the lower vent cove outside the van and feel the elements where they enter the burner tube, if one is hot, then the mains is working.
This leaves the problem of the gas seemingly heating the freezer plate instead of cooling it.
Whilst you have the lower vent cover off, look all around the gas burner box (on the right hand side below the burner tube) for any discolouration, especially a yellow or green staining.
If you have this discolouration then it is almost certain that the coolant has leaked and you will, depending on model of fridge need a replacement fridge or cooling unit.
Depending where you are there may be a good mobile mechanic who would check it out for you.