Do you guys normally put a 12 volt gauge somewhere to monitor the battery? In-line perhaps?
Yes, i use a plug in type , just to give me a general idea of "how's by battery doing" . I don't leave it plugged in all the time, i use it when i need to .
Google 'plug in,12v volt meter'
I would also use a hand held 'multi-meter' to test volts as well.
Check one against the other for accuracy. Good RV tool to have .
If you don't have a 12v receptacle just add one and run heavy enough gauge wire for the distance you need to run it.
Do yourself a favor and check the battery you plan to use. If it's been sitting discharged too low for a period of time, it might not be very good anymore.
A fully charged battery is 12.6/12.7 volts and I would not let it go below 12v (while voltage is being measured while resting).
If you allow a wet cell hybrid deep cycle battery to drop too low (below 50% or so) you run the risk of doing damage to the battery.
They need to be kept on a 'smart trickle charger' either indoors or out.
The below link has very useful info on the 12v system and batteries.
12 volt side of life