I think a microwave using electricity (on a “renewable tariff”) is better than using the diesel, even if it is a minuscule amount. I‘d consider using a diesel heater with an open window to be no better than using a patio heater. If someone wants to do the math, I’d happily change that.
The use of the heater, as Welshgas pointed out, is to keep you warm as you've cracked open a window to ventilate the van, not to help dry. I don't see any difference what time of year you use it.
I’d much rather put an extra layer on than even put the heater on in the first place.
That's your choice and won't affect the condensation, but why not use the heater supplied for both you & your dogs comfort? If you are happy layering up in the cold, then open more windows to ventilate further.
I suppose the best option would be a 12v demudifier, powered by the leisure batteries? But my question for the group is how effective is that?
I don't have experience of the 12v models, but I firmly believe that you get what you pay for and a mains powered (desiccant) de-humidifier will be the most efficient (cost & environmentally) way to keep your van dry once on hook-up back home. The leisure batteries will be draining while powering the 12v de-humidifier, which uses energy (diesel / hook-up) to recharge.
Silica gel bags work by absorbing moisture until they saturate, then become useless until they are dried, usually by heating them. They are silent compared to a 12v (or any powered device).
The volume of space and it's humidity level dictate the type / size of de-humidifier needed.
While you're away using the van, I would accept the condensation in the cold weather but try and keep it ventilated as much as possible. Maybe someone here has good experience with 12v models while in the van, but with opening / closing doors and the cold van surfaces, I doubt you'll be dry in the mornings.
Please, we're not having a go at you, just trying to help!