Originally Posted by Lfemat
No, I donīt have a bypass valve
That seems weird; I though most Airstreams had one, to separate the water heater from the rest of the water system for winterizing, so you can fill the rest of the system with antifreeze while leaving the water heater dry. Still, you should know your own system, if if you say you don't…
Next question… Do you have a mixing valve? I mean one right at the outlet side of the water heater. At least for the Atwood water heaters that Airstream uses in some of its models, it stretches a 6-gallon hot water capacity to 9 gallons by means of a mixing valve that mixes hot/cold at a 2/1 ratio, and makes up for it by heating the water to a higher temperature. All this even before the water gets to any faucet. Then, at the faucet, you can mix in additional cold water to fine-tune the temperature to your liking.
It seems like a problem with the mixing valve could explain the results you're getting.