New Chassis Battery is Dead

[Jim.B1]

I replaced the chassis battery with a new one on our 2014 Southwind about 2 weeks ago (Costco 850 CCA, size 27).
The RV is in our driveway and plugged into a 15 amp receptacle with nothing turned on in the RV, both battery disconnects for main and aux. off, and the Magnum Inverter off.
I have been monitoring the system and the main battery has never gone above 12.1-volts.
I went out today, (after not checking for about 7 days) and the main battery reading is 3.8-volt and no power.
The house batteries are fully charged at 13.2-volts and the generator starts with no problem.
I am looking for any suggestions as to what might be drawing the power, if I should be leaving the Magnum Inverter on, and could it be a fuse that is not allowing the chassis battery to charge.
If it is a fuse issue, any suggestions where I should be looking.
I am doing nothing different than last Winter in terms of storage, yet my new chassis battery is completely dead.
Thanks in advance for your comments.

[Jim.B1]

As a followup.
I have left the RV plugged into shore power on a 15 amp outdoor receptacle.
I have switched both Aux. and Main Battery disconnects to the "on" position and switched the Magnum Inverter to the "on" position.
After 15 minutes, the system monitor for the Main battery is reading 14-volts and the RV started with no issue!
I have determined that I "must" leave the battery disconnects in the "on" position when connected to shore power in order for the chassis battery to receive a charge.
I know I did not do that during last Winter's storage.
Now, I need to find the source of the draw that is killing my battery.
It "may" be how I have wired the 12-volt connection for the GPS as it does receive power even when the RV is not running.
AC/DC has never been my strong point, in turn, lots of questions, and trial by error.

[paddler1954]

Jim B. glad you found the error of your ways. The inverter must be on to charge the batteries. As to your parasitic drain, yes the GPS could be an issue, also pull the fuse on the radio. Some radios have a weather band and it is active even when the radio is turned off. I also have a parasitic drain I need to chase down, so I feel your pain.

[Jim.B1]

Originally Posted by paddler1954

Jim B. glad you found the error of your ways. The inverter must be on to charge the batteries. As to your parasitic drain, yes the GPS could be an issue, also pull the fuse on the radio. Some radios have a weather band and it is active even when the radio is turned off. I also have a parasitic drain I need to chase down, so I feel your pain.

Hi Steve,
Thanks for your input, much appreciated.
I have now corrected the GPS wiring so that it only receives 12-volt power when the ignition is on.
I will also ensure that I leave the battery disconnects and inverter "on" when parked in the driveway and connected to shore power.
Never too old to learn!