Is there a "fault" light by the switch? If so is it on? On my trailer, even with the switch on the burner doesn't come on unless the water has cooled enough for it to need heated again.
I think the fault light will come on if the burner tries to light and doesn't for some reason. Mine comes on when I fist turn on the switch, but goes off when the water heater lights.
Another idea of something to check.
2012 Flagstaff V-Lite 30WFKSS
2015 Ram 2500 Big Horn Mega Cab 4X4