Unless the old battery is draining the new as Chuck supposed and the
combiner isn't working or was installed improperly, then you have a
continuing load on your batteries. It must be a significant load of a couple
of amps or more to run down a battery overnight. The easiest way to find it
is with a DC ammeter. If you can borrow a clamp on type, you can find the
culprit in a few minutes.

Otherwise some multimeters have a 3 or 10 amp setting (usually requires
separate sockets for the leads) that you can use. Disconnect each possible
load and then connect the ammeter in series with it and check if it is
drawing any current.

Lacking either of the two types of ammeters and if the unknown load is
significant as I suspect, you may be able to check it with just a good DC
voltmeter. First disconnect all loads and wait 30 minutes. This will let the
battery build up to full voltage. Then with your voltmeter on the battery
terminals, connect each load one by one. The culprit will cause a small
(maybe as little as a few tenths of a volt) drop in voltage at the battery
terminals due to the load pulling down the battery voltage a bit.

David