Lots of good advice here: Al tanks, store tanks in lockers, out of bilge, etc. But I seriously suggest you talk with a reputable dealer/installer. There are a lot of nuances here, including installation specific issues like venting. Don't forget the propane and CO monitors.
I looked into a propane heater for my 22. It looks feasible, but is not something I would do myself.
I looked into a propane heater for my 22. It looks feasible, but is not something I would do myself.