nosebreaker,
I believe there is a compromise you and your wife can make and still allow you to both enjoy the lifestyle. One, as some have pointed out, is to use campground showers at times. Though, I know my wife wouldn't go for that.
So, first of all, if your tank based heater doesn't have BOTH propane AND electric simultaneous capability, see if that can be added.
If you're NOT connected to hookups, it is very easy to run down your water supply and fill your gray tank by leaving the water running during a long shower. If you're connected to hookups, pull the gray valve open and have at it.
"Shower styles" seem to differ amongst people. Since we had the motorhome since the kids were small, everyone in our family is familiar with shutting the water off "in between" actions. Based upon visitors we've had to the house (family, friends, friends of the kids, etc.) it seems there are many wasteful practices people get into. One example is keeping the water running while brushing teeth, for instance. Unnecessary. Keeping water running while lathering the shampoo. Again, unnecessary. I installed a shut-off on our shower head in the house, as well as one on the motorhome. I expect the family to conserve water in the house as well. It's just a good practice. (We are on city water, and under no water shortage. This is more of a lifestyle choice.) I'm not a "tree hugger", but I do believe in not wasting resources. Especially those I have to pay for! (Don't get me started on shutting off lights when leaving a room!) LOL
Anyway, as far as the tankless systems go, let me tell you briefly how they work, if you don't know. Whatever temperature you have them set for, they will attempt to keep the water at that temperature as it leaves the heater. On a city water connection, the pressure, flow rate and temperature are generally pretty constant while the heater is in operation. This makes temperature regulation pretty easy. Unless someone else in the house calls for water. Then the system adjusts, and this sometimes DOES cause a fluctuation in the temperature of the hot water while the system reacts to the change in flow. But the mostly consistent pressure & temperature of the incoming water generally yields minimal temperature variations at the point of use, in this case, the shower.
In an RV running from onboard water, the water pump generally cycles on and off with demand. Thus, the tankless system will be constantly adjusting it's heat output based upon the changing flowrate of water through the heating chamber. I believe this is what most people experience as a major problem with these systems on RVs. Now, a different water pump, or adding a bladder-tank to the system might help even out these pressure changes. That's just conjecture, though, and not based on my personal experience.
One thing that might also help is to add a small 2-ish gallon electric water heater near the shower connection. This will fill with the mostly hot water from the water heater and might give enough buffer so that your wife will get a steady stream of hot water. Here's an
Example from Home Depot. This will increase your effective hot water supply to 8 gallons, plus give a buffer zone and a second chance to heat water before it reaches the shower.
But, personally, I still think there is either something wrong with your current system, or a slight correction in procedures which could solve the problem much more easily.
In any event, I'd hate to see you give up the RV Lifestyle over this issue. It should be solvable. Personally, I don't think the on-demand system is the answer.
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.
