Would live to see a picture of your wood burner…Are you 100% against wood heating?
Your battery capacity is not very much, as others have said it will struggle to run anything electrical heating wise for a reasonable length of time.
My small wood burner is on low at the moment and it's a toasty 25 C in here while it's a cool 5 C outside in NW England. I'm not sweating yet. My batteries are running all my other equipment and I must confess I also have an espar D2 installed which sees most use when winter transitions to spring and the occasional daytime use here and there if I happen to be hanging around indoors.
The 50 amp lithium that is already in the van should be plenty for powering a diesel heater, I think. it should be a pretty low draw. For my install, The thermo king guy put the espar directly on the sprinter battery, so I switched it to an auxiliary battery. He said it wouldn’t kill the battery, but he might have been working more on bigger rigs.In all seriousness, as others have said, diesel heater. 2 day install under passenger seat. Can temporarily use cigarette plug/cable for power until you can run permanent.
Having someone check out a self installed Propane system is a reasonable option. I have done my own work on the heater and replaced the stove, checking for leaks with both soap bubbles and an electronic detector. Note that the electronic detector will sniff soap bubbles as a Propane leak. A certified Propane shop worked on my tank. They seem to be very helpful and know all the ins and outs. You could also ask what the applicable requirements are.LPG, I fitted a Gas-it refillable underslung tank to mine, 30l, can be tricky to find a source, there is only one outlet in my large-ish (500k pop.) city - Morrisons fuel station, some but not all of their sites nationally carry LPG, their website will tell you where. Someone has also put together a pdf of their locations, found it on google though. In summer that runs my fridge/cooker for 4 weeks depending on weather, the fridge uses the bulk of that. In winter a fill will last right through from late October to well into spring as fridge isn't required to be on. I installed it all myself and was meticulous about checking for leaks prior to firing up the gas powered appliances. I met a caravan service bloke in a supply shop when building it, he said he would be happy to give it the once over but I didn't bother in the end, I'm still here 4 years on.
I have all the safety kit fitted as per marklg states above, except the propane detector. There are 4 total gas drop out vents which also serve as air inlets when the fire is going full tilt, in addition to the 2 roof vents and side window if doors are shut.
I hope you get something sorted soon, I feel for you in this weather and winter hasn't really got going yet.