Paul ,,, does your rig have a 12-volt cigarette plug inside the trailer?
Mine does ... I also have a plug-in battery tester (I think I got it at JC Whitney) that shows the charge level ... that way, prior to hooking everything up ... ie not hooked to tow vehicle or shore power .. I can insert the tester and get a quick read of the battery state.
I do this before I hook up at the storage yard ... most of the time ... to see if my 3 watt solar panel
kept the battery fully charged while in storage.
You could also use it to check the battery charge when you arrive at the campground ... but if you don't let the battery "rest" ... you'll get a false "higher than actual" reading because your tow vehicle will have been charging the battery enroute.
I also use the tester to monitor battery state while boon docking.
Not professional. And the tester probably isn't right on the money with the true charge ... but hey, it works for me.
I'm not a EE like someone I know ... so don't take what I say to the bank.
But I'm a EEE major.
I ENTER EGGS EXCITEDLY !!!
Yes I do.