Hey Jon, I just got some crap in the mail about on demand Hot water heaters.

Its called the "Hot Man Extreme S/C propane heated portable shower", made by Zodi Products, possibly also known as Zodi Outfitters or Outfitting. I've tried their website, bong, no luck. You try it : www.zodi.com or you can call 1-800-589-2849 . It looks like 3 or 4 gallons, heats to 100 degrees in 6 to 7 minutes, portable, and cant be more than $125. No more often than you shower, this could be paradise. One more guess, no more than $89?
Hot Man at Large