1. Air pocket has no bearing on hot water
2. The standard tstat is either 130 or 140, depends on the year of the Atwood. EITHER tstat, you could NOT just shower on HOT only. You would be scalded. That points to a problem prior to this problem
3. Let it continue the heat cycle and see if the ECO trips and/or the pop off valve opens. No harm if it does
4. It is very easy to install a ECO in place of the tstat. That means the temp of the water could reach 180(ECO Temp trip). I would suspect you have installed the ECO instead of the correct tstat in the tstat mount. Doug