Could be a little insect mess in the gas line, or in the flue itself, you may have to clean the lines. If that is unsuccessful, then try pulling off the plug on the circuit board of the W/H and see if the contacts are clean. Go turn on the switch, then reapply the plug on the board. (BTW) do you have any 12 volt inside. If not, then check your fuses (if plugged in to shore power) or replace/charge the onboard battery (12 volt supply) if you are dry camping. (I know you said he has been staying in the unit for a few weeks, so I imagine you are plugged in to shore power.