Easy and cheap.
Put at tee under the sink on the cold line to the sink faucet. That new line feeds IN the on-demand heater. The OUT of the heater tees INTO the hot water line. You choose where to tee in. I would tee in before the line tees to the shower and sink. EASY. Hard part is I only recommend the electric on demand heater that runs on 120 Volts. You will need to hard wire it to a 12 guage or larger extension cord routed outside so you can plug it in at the power pole. Too much demand for an RV power system. Wait, thats easy too! Costs $200. Here's one:
$90 on ebay now.
Both these lines in my trailer are under the bathroom sink. Its perfect. I wouldnt even need to cut a line because I could just buy two tees and fittings, and stick them between the existing tees in there. That is also the location where my 30 Amp line comes in. If you have an electrician friend, he (or she) could easily put in a 20 or 30 amp dedicated service with a GFCI breaker - bypassing the RV service. I'd just run a cord through the floor and plug it in outside. These are VENTLESS. Your done. Set the temperature to your comfort with a dial on the tankless heater.
As far as the grey tanks filling up... Grey water can be drained by a bucket and dumped in rose bush beds. Your neighbors will complain but its GREY WATER. GOOD for the environment. BETTER than the stuff I will spend hundreds of dollars on next weekend to dump on my lawn so it gets greener than my neighbors lawn. It's Water, Soap, TRACES of Organic bits, Ammonia, phosphates, nitrogen, and oils. Sounds like plant food to me. I have a bayonet cap on my dump line that has a 3/4" hose fitting:
With this, I can drain my grey tank with a garden hose just like watering the roses! Gross? That's one opinion and only matters if its the camp host's. Probably better than what that neighbors' dog is watering my roses with - and certainly nicer than what it dumps on my grass!
Water's precious. And wastewater processing adds cost to camp fees. I fully encourage you to git er' done. I can even draw you a sketch of the whole setup if you need help to visualize this.