If you haven't done so already, replace the anode rod... your water heater is designed to have one.
They're inexpensive, and usually available at most RV dealers or stores that RV supplies (maybe even WalMart, etc).
Replace the anode rod as part of your annual maintenance routine. Also, clean/flush the tank interior at least once
a year (minimum)... more often if you live/travel in areas with hard water (heavy minerals).
Not everyone uses the same routine, but we drain the water heater after every trip.
Terri, the Chevy co-pilot