All trailers will drain the battery eventually. Even if you think you
have everything off. Often this happens in just a couple weeks to a
month. Completely draining a battery is very hard on it.
You want to avoid doing that if at all possible.
There are things in the trailer that are pulling down your battery while
you aren't home. The LP detector and the on board battery charger/converter
will both drain your battery when you aren't looking!!
For this reason many of us have installed battery disconnect switches.
We flip open the battery switch when we are done with the trailer and
that way only the battery internal resistance will drain it. On a good battery
the internal resistance won't kill it for at least a few months.
If you plug in your trailer the built in charger should be able to charge
your battery fully in a couple days.
If you have a decent digital volt meter and read the battery voltage while
it is not charging and nothing much is turned on in the trailer you can
sort of guess how full it is.
12.75 volts with no load (nothing on, not charging) is pretty close to 100% full.
12.00 volts with no load is pretty close to 50% charge and you want to
try to always stay above 50% for longest battery life.
I use an unscientific method. When we are dry camping (camping without
a plug in) and my water pump is running, if the pump slows noticeably
right before it shuts off, I know my battery is getting low.
There are battery disconnect switches you can install yourself.
One type goes right on the battery post. Check that you have room under
the battery box cover before hooking up but these are the easiest to install.
The problem is you have to take off the strap and raise the lid on the
battery box to turn on or off.
Go to amazon.com and look for battery switch.
Also Harborfreight.com and look for battery switch.
Autozone should have them as well.
Several of us have installed this one and think it's as good as you can get.
Amazon.com: Blue Sea Systems 6006 m-Series (Mini) Battery Switch Single Circuit ON/OFF: Sports & Outdoors
Of course you can pay a dealer to install a battery switch. It should not
take more than an hour labor + the cost of the switch.
All trailers should come standard with a battery disconnect switch but
they don't. The makers leave it up to us to foot the bill for that one.
Good luck and happy camping!