Jedtoo... sounds like you need to focus on the wiring harnesses since you already swapped out the board. Sharing with you the intermittent issue I had, my control board is located close to the burner unit and they left the wiring harnesses at their original lengths, meaning lots of extra cable coiled up and zip tied. I found that by isolating and wiggling one of the wiring harnesses I could get the system to come on. In my case the issue was at the burner side of the harness not up by the control board. I carefully unwrapped each harness and shortened each wire as well as replacing all spade crimp connectors (both male and female). I also double checked all connectors where they attach to the control board. That was 6 months ago and my intermittent issues disappeared and my wiring is all nice a tidy. Your coach is 9 years newer than mine so that may not be the issue but thought I would share.
One other thought. I have spoken with the AquaHot service department several times and while they have been helpful they also made me aware of a local guy who worked for AH for years and now has his own mobile repair business. I've spoken with him and he's very willing to talk through issues on the phone and provide recommendations. His name is Johnny Carrillo (970) 518-3085. Also very knowledgeable and helpful is Roger Burke, he has a forum dedicated to AH and is very responsive RV Hydronic Heater Forum
Best of luck with your troubleshooting.