When boondocking and just using the cabin batteries without the generator, we must be concerned about not discharging the batteries too much.
Well, what is too much; too-much is when you run the batteries really down and your appliances like your furnaces grind to a crawl. It is generally recognized that the batteries should not be discharged beyond 50% so that their useful life will be extended.
Unfortunately, the motorhome's status panel cannot give a good idea as to the discharge state. One convenient method is to measure the battery's voltage with a quality digital voltmeter (DVM) and compare against a chart such as below. If you can get a chart from your battery's manufacturer, that would be best. See chart for Trojan...........
Trojan Battery Company
To measure the cabin's battery voltage, it can be measured only at the battery terminals with your DVM or accessorize your Magnum Inverter with a $190 ME-RC Remote Control, plus a $180 ME-BMK Battery Monitor Kit. These are great additions if you want to go that route and they add a lot of programmability to the Magnum's charging sequences.
However, a low cost alternative (which I used) is the Volt Minder which costs between $25 and $45 (see photo). I found it to be within 0.07 volts of my quality DVM which is within 0.5%. It also has a settable alarm level, which will beep when the batteries go under the set voltage.
A word of caution; when using the Volt Minder or a separate DVM, the measurements must be taken at the battery terminals.
I cut the cigarette lighter plug off and ran two wires directly from the battery terminals. The positive side must have a fuse right at the battery. The wires can be 18 or 20 gauge speaker wire. Since there is very little current drawn over these wires, there is practically not voltage drops.
They changed the Volt Minder a bit and can be found at:
Digital Voltmeter | VoltMinder.com
Battery's seem to be low-tech, but they are somewhat complex and are affected by temperature and a long list of items. My tip isn't meant to go into this deep dark cave, but only to offer you a simple, economical and somewhat precise way of monitoring your cabin batteries discharge state; especially when you are boondocking.