You would need to replace the water heater from only electric, to the duel unit..... going to guess $400 for that at least.
Then you should be able to connect heater hoses off the engine water pump, like your car heater, you need the block hot water behind the thermostat, so your not burning $12 of gas to take a shower.
The manifold water line is suppose to be cool water to cool the manifolds.
Next problem is your only going to push clean water thur the heater with that little 12v fresh water pump, so its going to be a tinkle hot shower, and cold water.
So, cost vs. use?
They do sell this little solar shower thing in the boat store which is nothing more then a bag or water on a rope, that sits in the sun all day, and then has a hose on the bottom, or try the solar panels, to do the same.
Your limited more by the 12v water pump then the V8- hot water making machine.