Although I consider that the 750 was the best model we ever owned, I am obviously wrong about the location or even existence of a remote air temperature sensor. The 700 that we owned after that did have a remote sensor above the habitation door, like a button, which I was constantly trying to press. I have recently talked to current 750 owners who suggest that the air temperature sensor may be incorporated within the temperature dials (where you select via the inner and outer dials the water only or water and heating functions plus the temperature required) and as such am unable to comment any further.
Hopefully, someone else will post here soon with a clue.
Yes you are correct in as much as it would be within the remit of a gas safe engineer to remove and inspect the unit.
There is a UK Truma helpline number which should be able to offer telephone advise with regard to your course of action.
Sorry to have led you on a wild goose chase.
