Not exactly the answer you were looking for, but this might be another way to help charge your house battery with the alternator you have. If you have an isolator(you should) They have a typical voltage drop of about .7 V across them which effects the ability of the alternator to give a full charge to the coach battery. There are other ways, like continuous duty solenoids, but they don't isolate the battery rather change it to a parallel configuration for charging which isn't really good if you have a standard cranking battery for the engine and a deep cycle in the coach. There is an isolator from Hellroaring that has a very tiny voltage drop, still safely isolates the battery and has the capability of setting up a switch for combining the batteries which is handy if you starting battery is run down. You would flip the switch for about 15+ minutes to put a charge into the starting battery, flip the switch back to isolate the coach battery then start the rig.
At any rate, just wanted to bring up another option.
-Mike & Heidi
97 Roadtrek 170P "Taj Ma Trek"