Downsides to leaving ignition on without engine running (diesel)?

Question

I have a camper (motor-caravan) which can get really hot on a sunny day. Opening the doors allows flies and other insects in, and running the engine produces noise and exhaust fumes which are undesirable for other guests of the campsite. It seems to me that running the blower of the heating/cooling system should suck air in through a filter, it will find its way out through the roof vents and elsewhere. Thus I want to turn the ignition switch to "run" but not start the engine, and then set the blower speed to maximum.

Are there any negative side-effects to having the ignition switched on for a period of time (tens of minutes) without the engine running?

Obviously it will use battery power but in general I am hooked up to the campsite mains power so that is not a problem. In any case I guess it will take an hour or more to significantly deplete the battery (in general I will have driven for several hours so the battery should be fully charged).

I have been told that this can destroy the coil on a petrol engine with old (non-electronic) ignition, but my camper is a 2004 diesel Mercedes Marco Polo (Vito/Viano).

EDIT: clarifications in response to answers & comments:
(1) I prefer not to dive into the vehicle electrics to rewire the fan; frankly I'm looking for a quick-n-dirty solution. Thus I want to know if there are any "gotcha's" to my idea.
(2) Space in a camper is restricted, so carrying an extra fan is sub-optimal.
(3) The Marco Polo has two batteries; both are charged during driving and during campsite mains hookup. The fan uses the start battery while the fridge and most lights use the household battery. I realize an optimum solution would be to arrange to drive a fan (internal or extra) from the household battery, but that's too much like hard work (see (1) above).
(4) I was unable to measure the fan current with my 10A multimeter (can't get past glowplug warming) but assuming 5A it will take 20 hours to empty the 100Ah battery, and 10-15 minutes is certainly no problem.
(5) The heating/cooling fan only runs when the ignition key is in the 'run' position; otherwise I could just use the 'accessories' position.

Answer 1

You'll need to check how your inverter/shore power is set up - some systems will only charge the leisure battery and not the vehicle's starter battery.

Personally, I'd get a bigger 12v house-type fan and run that - you won't get a lot of airflow from the cabin fan, and it'll be noisier. I'd also look at getting insect screens for the doors/windows so you can have those open.

Answer 2

If you are hooked up to mains power, then a possible solution is to use a small inverter (approx 10A) or an "old" battery charger (newer chargers are "smart' and recognize the battery, so may not start to charge) to power the fan directly - otherwise the vehicle battery will get drained.