I am no electrician either, but here is what I would do before paying some $135 per hour.
Recharge both battery banks.
Turn the battery disconnect for each bank to the disconnect position.
Try to operate the various components run by each battery system to see if current is still reaching that component when it should be disconnected.
If all is shut down ok, turn on the chassis battery bank. Then try to operate features that would normally be run by the house battery bank.
If the house bank features work, but the house bank is "disconnected" there may be a battery isolator that has failed and the chassis battery is running the house features. (I have had this type of problem.)
If that tests ok, then reverse the disconnect status of the battery banks and test the chassis features to identify any chassis features run by the house bank.
If that test is good and the shop could not find a draw on the chassis batteries, I would suspect that the new battery was not fully charged when you installed it.
And if you stored the rig with the house batteries in the "connect" status that you lost whatever charge they had in the meantime.
The above would mean that your batteries and system are ok, but need to be fully charged and the disconnects used.
If you have already done the above, I can be of no furthur assistance.
Hope you get to a resolution.
1995 CC Magna #5280
C8.3L 300hp Cummins, 31,000lbs