Forest. The cab cig lighter should be connected only to the chassis or starter battery. The house batteries are separate. Even though 12.4 is a good charge for the chassis battery, I think it should be more like 12.9 to 13.2 full charge. Start you motor and check your battery panel in the coach. Both chassis and house batteries should go up to 14.1 while charging. If there is a solar panel, your house batteries should read 13.1 or 13.2. The chassis battery will remain at 12.4 to 12.9. The electronics of the chassis will draw on the chassis battery over time. That is what I am trying to resolve because my coach sits for a couple weeks at a time. It is not practical to do a quick disconnect on the chassis batter like you do with the house battery disconnect. On my truck, the cig lighter and 12V cig lighter will stay charged for a while but will eventually cut off. I have not checked my 12V cig plug on the Sprinter to see if it stays charged all the time.