chassis alternator not charging coach batteries

On our coach that solenoid is activated by the ignition switch OR the aux start switch - just like you are talking about doing. As long as the alternator will provide the additional current, I see no reason not to. Are you sure your's isn't supposed to be doing that already? On our coach we have replaced that solenoid several times. The main indication of failure is the loss of house battery charging from the alternator.