I too struggled with trying to get the on-demand water heater working with the shower, but made a significant breakthrough this past weekend camping at a local state park with full-hookups. There are several good threads on this topic and I implemented a few of the suggestions.
1. Adjustable water pressure regulator
- I bought this on Amazon and cranked the pressure up to between 55 and 60 psi. The pressure regulator wasn't very precise in it's adjustment so this is where i landed by cranking it up as far as it could go.
I guess the theory here is the higher pressure forces the water to flow through the system faster, therefore, preventing the system from over heating and auto shutting down the heating element. I believe this is what causes the hot/cold cycles. Anyway, the hot/cold cycles went away for me.
2. Adjust the hot water temperature
, such that, it's set at a slightly higher than comfortable temperature that let's you only use the hot water side of the shower faucet. In my case, opening the hot water side all the way actually produced warm water (not hot), but had lots of good pressure. As i turned down the hot water faucet handle, the water temperature increased. I did this until i got the right temperature i wanted. I then proceeded to take a perfectly hot 20 minute shower.
3. Oxygenics shower head
- This was the first item I replaced when trying to figure out how to make everything work. I'm not sure this contributes too much to the hot water heater issues, but it take what little pressure there is and produces a great stream of water.
Overall, I'm super happy with the results of these changes. In my case it work for me, your mileage may vary. It's also worth noting that my dealer got approval from Jayco to replace the on-demand heater with a 6 gallon traditional heater. We don't do much boon docking, so I'm not sure what i'll do here.
Adjustable water pressure regulator
On-demand temperature dial
Oxygenics shower head