Here is what we have which is somewaht close to what you are asking:
Isotemp SPA 15L (4 Gals) w/ 750w AC heating element and a custom 200w DC heating element. Water heater is mounted in the cabin and has additional insulation around most of it. I am not
utilizing the coolant heat exchanging loop which the water heater has. We have a 2200w inverter and 480 Ah battery bank. 7.3 w/ dual alternators. 300W of solar.
The 750W AC element is run via the inverter when we are driving and takes just about 1 hr to get the water to 130 F. I am guessing the coolant loop would take about the same or longer, given time for a diesel engine to heat up, so there is no real reason to connect the coolant loop, the additional valves/plumbing, and the accompanying risk of scalding hot glycol/water from 200 deg plus coolant spraying around inside the cabin plus a leak which would potentially disable the van.
You can find watt calculators and liquid to liquid heat transfer calculators on the internet which will give you a very good idea of how long it will take to raise the water by X degrees given watts of input. It is a very linear relationship and 750 watts is about 4 times faster than 200 watts. 750 watts will raise 4 gals of water by 60F degrees in about 52 mins w 90% efficiency.
6 Gal will take 78 mins to get from 60F to 120F
Size of Tank: A trade off between usable hot water, time to heat the water, and space to be taken up by the tank. I found 3 gals to be a pretty good size but could not find a tank that worked, so 4 gals was the next best. But it depends on what the water is being used for. I have left a filled 5 gal solar shower on top the black bumper box and it is really nice very warm water for a shower and it is all the same temperature, but it needs to be sunny and it takes a few hours to get to that temp.
Mixing Valves: This turned into a bit of a surprise for me. The IsoTemp hot water tank mixing valves manage by selection of volume - X parts from the Water heater Hot outlet and Y from the water heater Cold Inlet tap. It is fixed regardless of water temperature. It is NOT temperature compensated. The mixing valve is designed for a continuous heating scenario where the engine is running and/or AC is continuously supplied to heat the water tank. The thermostat is sensing temp water drop and maintaining constant heating of water as needed. That is NOT our typical scenario which is stopped, power off, draw from the Hot water until it is cold. I will be bypassing or severely limiting the cold water mix at the heater mounted mixing valve and using the faucet as the mixing location. This way I can maximize the amount of hot water that can be drawn from the tank with no power being applied.
The DC element provides a bit of a buffer by using excess solar energy during the day and/or a 17 amp draw at night if needed to heat water for a short period of time. It definitely heats the water up just takes longer than the AC.
Mounting location: I wasn't sure from the way you wrote what you were looking to do if you were mounting this all outside under the van. If so, you will be in a greatly worse off situation. Your ambient temps will be much lower and if you ever head to freezing locations you will have to drain everything.
I have been giving some thought to what, if anything, might be a better solution. I love the small propane On Demand hot water heaters but do not want mounted inside. I have been on boats and in small houses with those and they work really well. Maybe something along those lines. The flat plate heat exchanger with an Espar D5 might be another way to go.
Interested to see what you come up with and what your experience is!