Marc,
Rudy may jump in here and give you better information, but here is what I have found....
If the AH system is working properly, you should not have trouble getting and maintaining hot water (regardless of the outside temperature). (Of course that is a big IF)
To answer your question directly...here is the relationship between input temperature and how many Gallons per minute you can consume while maintaining 115F water at the output. AH recommends no more than a 1.5 gallons per minute shower head.
In my experience, the key to having plenty of hot water is having enough antifreeze in the tank. I know that you said you have plenty, but if you are just reading off the bottle, you may get a wrong answer. Use a piece of tape and mark the level on the bottle, then turn off the electric AND diesel heating. Let the tank cool down for a few hours. If you don't see the level change, then you probably have a leak in the hose that goes from the radiator cap to the bottle. Once it is cool, carefully open the radiator cap. With a ruler or some similar device measure the coolant level. I suspect you will find it low. Top it off with DISTILLED WATER.
Also, your mixing valve may be broken and only allowing cold water through, but I would not think that would change with the amount of usage.
In the short term, you could convert to using tank water and turning up the bay heaters to "pre-heat" the water.
Here are the things that were wrong with my AH when I bought the coach...
- Leak in tank about half way down allowed coolant to constantly leak under coach
- Leak in neck of tank where the flange attached to hold the radiator cap
- Leak in hose that went to bottle
- Bad mixing valve
- Bad shower temperature select cartridge
- Webasto blower motor was bad and needed to be rebuilt
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