THat happened with two of my motorhomes, and I installed a new relay. I did not bother to locate the original factory one the last time (it is buried in a box with many large wires) but instead just hooked up a new one that does the job, the factory one with it's worn out contacts still is in there providing some charge ability.
I used a forklift relay #6C017
http://www.grainger.com/product/WHITE-RODGERS-DC-Power-Solenoid-6C017?searchQuery=6c017 from Graingers.com
I hooked up some #4 wire from the alternator output to the relay input to the battery supply +12 wire. This transfers the highest possible voltage (alternator output is about 14.5 volts, VS the 13.5 volts near the engine battery) directly to the coach batteries.
Fred.
