Hi there,
There are two companies that make the best ones out there and I would advice you to look for these rather than anything else. You probably know them by now.
-Webasto. Webasto ones work on diesel.
-Truma/Trumatic. Trumatic ones work on gas also known as GPL.
Based on your requirements I would say that the Webasto is the one you need.
Now, why do I say to look only for these two? Because of safety and reliability. Here are the arguments:
-First of all, these are truly tested and well thought. All campers come with one and people that are self building campers usually fit one anyway. Some commercial vehicles also come with Webasto system already installed.
-Safety. I would not trust any of these systems with chinese writing on them from ebay. Nor from a safety or reliability point of view. Things can go wrong from many things, from gas leaking in the cabin to other many things, potentially very dangerous.
-Reliability. If you connect such a system to your gas thank you would most likely like to forget about it and the unit to keep running for years. These are one of the "fit one time" thing and repairing or fiddling with them can be a huge pain in the..
I would not recommend one with an auxiliary tank. These, as Carol said, can spill and can be as well dangerous. Imagine having diesel spilled in your truck and for how long that would smell. It also takes really valuable space. Also, gas nozzles aren't that long so you will have to take the auxiliary tank and fill it out every time you need to do so. Remember that this needs to stay very well in place so it isn't as easy as just picking up a plastic recipient and filling it with gas.
You can look for webasto systems at the scrap yards made for campers. They don't need to be new. Mine is more than 10 years old and still keeps going. Look for ones that come with a guarantee or are tested and it is proven that they work.
Now, I don't say that getting this won't work but personally I wouldn't take anything else but the two systems mentioned above. Safety and reliability first. Money come and go..
There are others ways that can keep you warm too like isolation or the plain old sleeping bag. This could lead, sadly, to condensation. I have a sleeping bag that is rated to -15 celsius and I've slept outside, just on a Therm-A-Rest Z Lite in shorts and a t-shirt at 2 celsius with no problems, actually I was a little bit warm so sleeping bags can be a very capable and cheap solution.
If you are interested in talking more about this or looking for other informations let me know.
Thanks,
Paul
1999 Ford Transit Smiley 2.5 Turbo Diesel, manual gearbox, transformed to a camper.