I'm thinking of an exhaust pipe heat exchanger. Maybe I can coil some copper tubing around the exhaust pipe near the end, right in front of where my muffler used to be.
Honestly, I'm thinking this might be the best idea here.
IMHO, it would be best to keep the hot water system and the cooling system separate. Not to mention, even though you have an external tranny cooler, the residual tranny fluid in the cooler is an issue.
Since this is not going to be a closed, pressurized system, the matter of corrosion comes into play as well. Granted, it would take a while, but not something I would want to risk.
I would go to the local hardware store, HD, or Lowes, and get some copper tubing. Make a worm out of the tubing, going from the end of your muffler to the O2 sensor after your cat. Try to keep the coils as close together as possible, increasing the surface area of the tubing, allowing the heat from the exhaust to warm up more water.
Run it back to the storage tank, or a different tank, if you want a hot and cold water set-up.
You can also get some wider diameter exhaust tubing, and cover the copper pipe to protect it from the elements/obstacles, as well as increase the SA of the heating surface. Just weld it to the flanges on the exhaust after you've covered the pipe in copper tubing.
All you would have to do, depending on how big your tank is, is run the pump for the tank for the last 30-60 minutes of your drive to ensure you've got hot water.
I would advise, DEFINITELY running some tests to see how hot that water would get from your exhaust. You don't want to scald yourself.
FWIW, this is the plan I am going to go with, should I ever decide I need hot water. (If I could get the wife out camping with me.)
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