Is the board attached to the water heater itself? Could you take the screws out that hold the water heater to the camper and add some type of calk behind the whole water heater? Maybe the whole thing is acting like a megaphone and transmitting the sound back into the cabin. Does the water heater set tightly on the floor? Maybe something slid underneath the heater itself might help? Maybe not, I'm making guesses.
The water heater I had with the noisy board just had the board mounted behind it screwed to the floor. There was enough wire in the case to mount the board to other places near the heater.