The problem is that i have a tri-plex line that runs from my brother-inlaws house to a dairy barn that is my shop and honey house. I am trying to cut his costs. That is why i am going with propane.if you are going to only use it a few times a year put in a cheap electric one. the tank-less is best if used everyday. it is money to spend that the fuel savings will never pay for in your situation. not using it for long periods will cause more not less maintanence. drain the electric in the off season, shut off the power when not in use, remember to fill and turn it on the day before you use it.
if it gets real cold outside a 20 lb. cylinder will not deliver vapor fast enough, you will need a larger tank.
John i am not following you. ( A small tank will freeze up even inside @ 90*. )Tankless heaters operate from 50,000-140,000btu for the small units. A small tank will freeze up even inside @ 90*. I recommend at least 120 gal tank.