Waste oil burner might be an option for heating...use any old engine oil for fuel...
Alternatively you might do better with a space heater than a calor gas heater...they use more gas but you would need to run it less, and it more importantly gives you air movement.
The other suggestion I have is to reconsider how you are drying...what I mean by this is that if you are just running a heater in a caravan then this isnt going to be effective or efficient. Your heat will all disappear out of your ventilation, if you have no ventilation then you will just trap the humidity in, so things wont dry effectively as the air is so humid in there.
I would be more inclined to consider air flow, and get lots of it...and some means of extracting the moisture via a condensor of sorts. Basically turn your van into a giant condensing tumble dryer (but without the tumble action!). Static heat is great for cooking, but no good for drying...to dry you need airflow and a means of extracting the moisture.
Put simply I would put in a duct with some fans that will give you plenty of airflow of hot air around the van, but dont duct this to outside or you will pump all your expensive hot air out...pass this hot air which is heavy with moisture over a cold surface in your ducting and the moisture will be deposited on this surface...This surface can be hollow and kept cool by a cool air system which ducts air from outside the van through it and then back out of the van using another fan..then fit some sort of drain so that the water runs away from your cooling plate to outside or into a container. This is basically how a condensor dryer or low bake spray oven works...your system at present from how I understand it is more like how your cooker works.
To get more effeciency I would insulate the windows, and anywhere else that attracts moisture at present, as the moisture is not collectable and extractable from these points.
As a quick fix in the interim until a better system could be installed, you could look to add a couple of fans in there to give airflow, and run a dehumidifier to extract the moisture,
Or simply duct a space heater into the front, and an extractor out at the rear...granted your heat is going to go out of the extractor, but so is the moisture...This will work like a conventional tumble dryer as opposed to a condensor dryer, but I would think even with the increased gas usage of a space heater, your overall drying cost would still be significantly cheaper as it would dry much much quicker and better as you are increasing airflow to pick up the moisture and positively extracting the moisture rich air.
Hope this helps mate...