Hello David.
If you trawl through these pages you will find several references to thermostat problems. The general solution is to have the thermostat repositioned, and there is an extension lead available to allow this. Contact your supplier to see if it can be done under warranty, failing that speak to the UK distributor for the heater (I assume it is a Truma) I must point out that the these fitting faults are the responsibility of the caravan supplier not the heater manufacurer.
The other issue is as you rightly say is the length of hot air ducting which is under the caravan. Some UK manufactures do a similar thing, but they take the trouble to insulate the duct to minimise heat loss.
As I recall, Truma do supply special over ducting which provides a waterproof jacket and some thermal insulation, It slipd over the normal hot air duct.
The other small possibility is to re-route the ducting around the interior of the caravan, but there is an additional problem that it may require a lot of addional bending to make it fit. As any aerodynaics expert will tell you each time you bend or extend a pipe you introduce more drag, so the air flow will be reduced. It is best to keep all ducts as short and as straight as possible, so the underfloor can sometimes be the only practical solution.