3. After starting the C-13, what was the chassis batteries' voltage reading? It should have been 14+ volts if the C-13's alternator is working. If not 14+ volts, then your C-13's alternator is not working. the voltage guage [gauge] shows over 14. But the Aladdin was in the 12's or lower
Our Aladdin's are probably similarly functioning, if not identical. On my Aladdin screen for reporting electrical stats, screen #3 I believe, the reported battery voltage reading pertains to the house battery bank. On the Aladdin screen for reporting engine and transmission stats, screen #2 I believe, the reported voltage reading pertains to the chassis battery bank. My guess, from your answer, is the C-13's alternator is throwing a charge to the chassis batteries (as evidenced by the 14+ volt reading) but the "battery combiner", probably a BIRD/Big Boy system, is either not functioning or has not kicked in yet (as evidenced by the 12 volt or lower reading). My suggestion is immediately after installing the new batteries, you should verify the "battery combiner" is functioning properly. Otherwise, your new batteries will soon become discharged.
When your coach is plugged into shore power or its generator is ON, then both battery readings, on Aladdin screens #2 and #3, should be the same - after a charging delay, they should come to rest on 13.4 to 13.7 volts. Conversely, with the C-13 running, both of the Aladdin battery readings should come to rest on 14 to 14.2 volts.