I am currently using an under-sink 2-1/2 gallon water heater in the kitchen, which I am going to put in my bathroom. Lowe's has one for about $120.00.
In place of this heater I am looking at a Zodi unit that I can tie into my current propane system so I can have on-demand hot water without shore power. They come with a 6v battery pack, but I already have a 12v Shur-flo pump that I'm going to use instead.
I don't know how much continuous hot water you need, but some of the Zodi models do not take up much space and, well, they're continuous!
Check them out at http://www.zodi.com
. (Sorry, I don't know how to put the link up here.)