Amanda supplied a link to a small portable water heater that could come in real handy in some places.
http://www.heatershop.com/coleman_hot_water_heater_2300_700.html
In general, things that can be run off a 1-pound propane bottle have a built-in regulator, and so can just be hooked up to a grill-gas tank with one adaptor or another--either direct or through one of those poles that has a couple of places to fasten onto. That would give you "up to" a hundred gallons of hot water without having to replace the tanks.
Instructions all say that the 25 pound (grill-gas) bottles should not be used indoors. Probably true, especially if they leak, but they just go "poof" presumably with a jet of fire but don't explode if they get too hot. (there were two within range of the fire when my house burned down. I heard them go. The firefighters didn't turn pale when they heard about them.)
I have had set the tanks outside with the hose coming into the building.
I'm thinking that a 1 lb container of propane gives you up to 40 gallons of heated water-- a 5 gallon or appx 25- 30 lbs should give you over 1000 gallons of heated water.
Whats neat about this is that it is about $160 and I believe the major parts are guaranteed for 10 years.
I didn't see flow quantity information,
sometimes I really can amaze friends and acquaintances with my in-the-head math.
sometimes one way, sometimes the other!
Sam's Club sold Coleman water heaters as seasonal item last summer... perhaps they will have them again this summer.