Hello David,
No! you are not going dotty1
For some reason, some caravan manufactures decide to fit the electronic thermostat for the space heater in a place where it received a disproportionate amount of heat from the heater. This keeps the T/stat warmer than the bulk of the caravan which causes the heater to think the caravan is warm enough, so it turns down the heat output.
Repositioning the t/stat to a better position restores its function and the caravan will be allowed to warm up better.
If you have the problem, speak to your dealer. In some cases the caravan manufacture has sanctioned the supply and fitting of an extended thermostat sensor.
This is not the fault of the heater manufacturer; it is down to the caravan designer who has not specified the correct placement for the thermostat.