Can I fill a liquid propane tank myself? (for household heating)?

I have a 120 gallon liquid propane tank outside my residence. My landlady owns it but she bought it so long ago that she's lost proof of purchase. I want to use LP to heat this winter (I've used electric since moving here) but I can't afford the minimums required by the LP companies here. I've seen small LP refill tanks for sale. Is there a way to buy them and fill up my outdoor tank with them? Or can those only be used for grills? I'm very good at conserving and can heat my home with my 10000 BTU LP heater BELOW its lowest setting. I don't understand much about how LP heating works but it would be a financial blessing if I didn't have to pay for a huge "minimum" amount upfront. Are those small tanks an option?