Well, my $0.02, I don't have one in my motorhome, but I did install one in our sticks & bricks. Whatever it saves in gas/electricity, it uses in water while you're waiting for the hot water to get hot enough and travel through the pipes. If you're always connected to full hookups and water conservation isn't an issue, I guess they're fine. If you're boondocking a lot and water is at a premium, I think you're better off with a tank-based system.
Just out of curiosity, does your water heater heat with gas AND electric simultaneously? If so, do you have both on when she's using them? If so, and she still doesn't have enough hot water, I would think it's time to re-evaluate her procedure. Tactfully, of course!
🙂 We have a 10 gallon hot water tank in our rig, but with gas & electric running, we have never run out of hot water during successive showers. It does require that you shut down the water flow while soaping up. If your shower head doesn't have a shut-off installed inline, that would be my first suggestion.
Good Luck,
~Rick
2005 Georgie Boy Cruise Master 3625 DS on a Workhorse W-22
Rick, Gail, 1 girl (27-Angel since 2008), 1 girl (22), 2 boys (23 & 20).
2001 Honda Odyssey, Demco Aluminator tow bar & tow plate, SMI Silent Partner brake controller.