Paul,
There are several things that could be causing your issue, but before you start throwing parts at it, I would contact ITR (the manufacturer) for a little troubleshooting help.
5305 NE 121st Ave, Suite 401 Vancouver, WA 98682
Telephone: (360) 993-4877
Toll Free: 1(800) 993-4402
It sounds like a control wire, thermostat, or control board issue, but a through diagnosis of the system should be ran before you start with the usual RV repair shop method of throwing expensive parts at it until it works. The Hurricane heater is a simple system that is much easier to maintain than a Aqua Hot unit, but it can be a little bit difficult to diagnose at times.
Gerald