Welcome Dragonfly, The shortest answer to your proposed design is, Yes. Beyond that there is probably a lot that can be discussed. Your post caught my attention because my van happens to be a RB, PH top, 4x4, no propane, with an esbar diesel heater that I had installed a 12v hot water heater and aft door shower. However I live in the Springs which is only 6100 ft. Oh and it's not a sprinter. But!
So, my van originally came with the flat plate that SMB used in 2005 which when I bought the van used in 2010 had ruptured and mixed the coolant and the domestic water. Long story for another day. So I haven't had hot water for several years but wished I did. I was inspired by a post by forum member that had modified a point of use 2.5 gallon water heater with a 12v element. Sorry I can't seem to find that post. So here is an over view of the system.
Home depot 2.5 gallon water heater installed in back cabinet. Cold feed line comes from under the sink just to the left. I removed the original 1500 watt element and replaced it with a 300 watt I purchased from here 6 Inch DC Submersible Water Heater Element
. Hot water goes to the sink and the shower faucet on the other side of the cabinet. I wanted to have the shower pipe short as to have the hot water nearly instant. Yes, I need to run a copper pipe from the T/P valve out under the van. It's on the to so list.
Here is a view of where the water heater is located (same open cabinet as previous picture) and where the rear faucet is located.
This is shower mode. If privacy is wanted I just clip a tarp/curtain across the back doors. I have one of those portable aluminum RV steps so one isn't standing in the dirt.
Here are a couple of thoughts though. I have only one 4D 200AH house battery with 280 watts of solar so, not a lot. I do have a 200 amp alternator with dual rectifiers so I do run the water heater any time I'm driving. If the water heater is cold say 60 degrees it dose take 1.25 hours to get to 126F ish. I usually see on the batt monitor that to go from cold to hot will burn 30 ish Amp hours if I try to do it all off of the battery (which I hardly ever do). If I have lots of power available, say solar is topped out and its 1PM on a sunny day, I will "dump load" into the water heater. 2.5 gallons of 126F water isn't a lot so no Hollywood showers however Mrs. ShuttlePilot and I have taken consecutive showers conserving. Traditional RV folks love to see my van and then flip out when I tell them I only have 2.5G of hot water. Finally, the water heater in the cabinet will keep the water hot for a pretty long time. I have run it at noon and still had fairly hot water in the evening.
I recently spent 3 weeks living out of the van and used this shower setup most of the time. I wouldn't say this is the best system for everyone but it's worked way better than I originally thought it would and no more drinking antifreeze. Oh and I think I only paid 200$ for the water heater, 30$ for the element but I splurged on large cable to wire it, don't remember what I paid for that but voltage loss is your 12 volt enemy. Plus some pex and fittings. So Yes it is possible.
If I can be of assistance please ask.
Have great adventure