I want to be able to wash hands and dishes with hot water, no showers.
Teresa,
BJ and I have had our 04 Monterey for about a decade, lived in it full time for over 2 years and continue to use it about half of every year. You should have no trouble washing hands and doing dishes with electric only.
I'm writing the rest of this assuming you leave the electric hot water switch on 24/7.
We both use the shower daily. Our habit is to turn on the diesel for the duration of the shower. I can shower on electric only but am lazy, slow, not green, wasteful and like luxurious showers. BJ finds getting the shampoo out of her hair takes long enough that an electric only shower is not a good idea.
An important rationalization is that HydroHot maintenance costs will be a lot lower with regular use or "exercise". If a little fuel doesn't flow through it frequently, things get clogged up, stuck or quit.
How much fuel does it take? Orman is correct if it burns continuously. Even in chilly weather, using it for cabin heat, it cycles on and off. Mostly off.
It is difficult to measure precise use but you can come very close. Our diesel fuel gauge remains pointing to the quantity in the tank at engine shutdown. The next time you turn the key, the needle will move to the current quantity. If you stare at the gauge the moment you twist the key you will see it move. The distance between the 1/4 marks is surprisingly close to 25 gallons. If the needle moves by one "width" it is about a gallon. You don't have to start the engine, just turn the key to "on" once a week and you'll know within a gallon or two how much fuel is being used by your HydroHot, our generator if using it.
At the Boot Scootin Stock Yards rally we chose to run the generator continuously so we could keep the windows closed, the dust out and the dog cool. The needle "jumped" about 12 gallons when I turned the key.
My best estimation is that turning the diesel on for showers and dishes only consumes about a cup a day as long as the electric is on continuously.
All of that said, I think your HydroHot is not working correctly. For two or three of our ten years, our hot water performance was marginal. We bought it new and knew what it should be like. Standard household temperature is (I think) normally set to 125 degrees. The HydroHot has a bunch of parts in it that should provide a steady 125 degrees for long enough wash the dishes and certainly your hands. Turning the diesel on to get it hotter only means one or two parts out of the bunch aren't working well.
During our two or three years of crappy hot water performance, two HydroHot technicians, that I like and and respect failed to find the problem. At the Bend Beer rally last summer Jerry Carr arranged for John Carrillo to provide service to several of us at a group rate. John was busy for two days.
After describing our issue, John advised that it would take time to do proper trouble shooting. We agreed to pay for the time it took. He measured and tested everything. He had one tool I'd never seen before. After about an hour, maybe longer, John said that he had tested everything testable. The last part on his procedure could appear to be functioning even if it was not working at full capacity. It is called the "stir pump". The only definitive way to find out was to put in a new one.
It might be interesting that the "new" one did not look like the old one or fit where it was installed. I know ours was replaced once at about year 3. Now twice. I think I understand that AquaHot had to make a change due to high failure rates.
As soon as John replaced the stir pump, we had hot water just like in a stick house, like it should be. Although I didn't notice it much when the stir pump was not working, the diesel had to run longer for cabin heating as well.
John left me with the old stir pump. I remember when the first one broke, I took it apart and found the magnetic clutch drive inside was in pieces. This time I hooked up the wires and some tubing and found it was pumping, but not much. I think the magnet had weakened.
The stir pump is not cheap. Nothing for a HydroHot is cheap. But, I'll bet a beer or a cup of coffee you need a new one. We've had two fail and the replacement is a different design.