Is the portable unit you're contemplating, AC or Propane fueled?
Reason I ask is there's a really old travel trailer a few doors down from me. It's in really bad shape. I ended up helping the older woman who owns it and her family, set it up after it was delivered. I'm being kind when I say it was really old. It's actually in poor shape in all ways imaginable.
Anyway - I noticed that where the water heater door was, there was a hole cut in the door and 2 pieces of 1/2" PEX sticking out the hole with some 90 degree elbows on each piece and some 1/2" fittings on the ends.
Later, we found a portable, 120 VAC water heater on the floor in a corner of the RV. I can only assume the previous owner plugged this water heater into an extension cord to power it, set it up in a short table or a chair, outside of the unit and connected it to the extended plumbing.
My point being - seems that your idea has been done before. I've had propane fueled tankless water heaters before but I've never used or even researched a 120 VAC unit. Unsure how fast they recover or what size their tanks are. This unit was rather small, so I'm guessing maybe 3 gallons.
Keep asking questions, keep researching and consider posting on other RV forums as well - perhaps IRV2 - to reach an even wider audience.
