Just reread your original post. You said that the voltage reads 12.9 volts with the switch on both and 13.2 with the switch on 1 or 2. Where are you measuring this voltage from? The alternator output wire should go to the Guest switch common. From there it would go to whichever battery you have selected (or both). There may be a smaller wire going to the panel if you have a voltmeter. But it should not be the only output wire. I just finished adding a new engine panel and rather than shortening the harness
, as I moved the panel 6' closer to the engine, I rewired the engine totally instead. I found that the original wiring (32 years old) was not tinned marine wire like what we use today and there was evidence of corrosion as well as it not being of a large enough size. My guess is that somewhere from the alternator's output to the Guest switch or from the switch to each battery there is more resistance than you want - there should be minimal resistance. This could be either from a bad or loose connection or the wires themselves could be suspect if old. The ground wiring could also be a problem - either the alternator connection, engine ground, or battery posts themselves. If there is more than one issue with older wiring replacing may be a good idea.