[A/S] Hot Water Problem

["dlombardozzi" <dlombardozzi@xxxxxxxxx>]

My wife and I recently purchased a 1991 25' Airstream Excella.  On a 
recent trip to Tennessee we came across a problem with the hot water 
unit.  The switch for it to ignite is in the bathroom - it works 
fine.  The unit lights without any problem, however, the unit fails to 
re-ignite to keep the water hot.  On one occasion it even failed to 
shut off the propane and kept heating the water until it opened the 
pressure relief valve and spewed out boiling hot water (it only done 
that just once).
Is there a possibility that I have a thermostat problem or is it 
something else?  If it is the thermostat, how easy or difficult is it 
to replace?
Any help would be very much appreciated.
Thanks.

Danny