Custom Water Heater, anyone

[edco]

Maybe I'm crazy, but I want to install a small hot water heater in my '88 E-150 SMB. Thought it would be nice to have warm water without having to fire up the stove. This is an interesting situation for all you do-it-yourself folks out there. I have extremely limited space to give, so it must fit under the sink (see picture attached). Def a custom job, I'm thinking..on semi-limited funds ($250?). I already detached the town-water supply hose (would never use that anyway), so I have the hot water feed on the sink faucet freed up. I also found a nice little water heater element with a 1" NPT thread for $75 on-line. The website says if you get the 600w/24v model and run it at 12v, then it will only draw 150w...which is 12.5 amps by my calc...correct? I'm thinking this would be a 4 gal. (max) hot water tank, so it shouldn't take much time to heat the water (1/2 hr? just a guesstimate, maybe less?). If I heat the water when the engine is running, I won't have much battery drain, and put in a thermostat in case I forget to shut it off. The bigger problems are squeezing something into the small space next to, or above, or around, the existing 10 gal cold water tank, and figuring out the water flow to feed both cold and hot water to the faucet. I was thinking of a system that automatically fills the hot water tank from the cold water tank, so it never runs dry...a pressurized system would be nice, but not absolutely necessary, unless it IS necessary to accomplish what I have planned. Prob have to have a custom hot water tank welded up? Maybe double-walled so it keeps the water hot and doesn't heat up the compartment too much and melt the nice fake wood veneer? btw, there is only 5 inches between the existing tank and the wall...yoiks.
Anywho, I think it can be done and would be a sweet addition, but I don't have any experience...just a lot of imagination. whaddyathink?