Never figured the cost to run the heater cause i really don't care..keeping warm is the important part, not the cost...as for how long a 16 tank last, just depends on how often you have to use it...sit in Racine wisc in mid jan for a few days, you will go throught both bottles quick as you will have it on all the time...it also depends on how hot you want it...the BIG buddy has 3 different settings 4000, 8000, and 12,000 btus so the hotter you want it, the more it will use...I know that u 1 lb bottle set on 8000 btus will last 4 to 5 hrs...and the usage also is affected by how much OUTSIDE air gets into the van...the more outside air, the longer the burn...
As for the gas usage on the generator, I don't use the gas tank on the generator at all...I disconnected it totally and use a 6 gallon external tank that i connect to the generator with "quick disconnect fittings" and i will be changing that over to connect the generator directly to the van fuel tank before next summer so i will then have up to 33 gallons to run it on...
As of right now, I have run the generator for up to 20 hours off the 6 gallon tank and still had gas left....i figure i can get around 30-36 hrs if i pushed it...