I find a ton of confusion and loose talk that loeads to even more confusion!
When they tell you not to leave the batteries connected that is speaking loosely!
We have to leave them connected to charge. We also can leave them connected when we are using the RV and connected to power.
What we get into trouble doing is often a matter of how long and what the charging does as welll as how we watch the RV.
Good battery care depends on several points which are a combo of the equipment and the user.
We want to both keep the battery at the correct "float level" and also not run it too hot nor too low on water. So to find what works for each RV, we need to know about what the equipment will do and also what WE will do.
Newer, better charging systems have three stages of charge. If the battery is way down, the charge goes into bulk charge, a higher level to get the battery voltage back quicker. This stage should taper off as the battery gets to it's correct level and finally as the battery gets just slightly above the norm, the charge should taper off to a float level. This should be the best level to keep the battery long term but is foten slightly higher than the 12.8 a standard lead acid battery.
When we get a new RV or new batteries, we need to assure the RV is treating them the best we can. If we leave them with charge on, we need to verify that the charging is doing the three stages as one big item! That takes checking frequently to assure it is not too high, nor running the battery dry of water. Both are fatal to battery life!
That means if we want to leave the charge on, we have to work at watching what is happening and that can be somewhat easy or impossible. How we each live and use the RV, changes what is needed.
If you are in a situation and mindset, it can work well to leave the charger on, check the voltage and batteries frequently and do the time and effort to keep them good.
But that means way past what many of us are willing/able to do for time and effort. cleaning and maintaining batteries was one of the big things that made me retire from telephone work! It is NOT fun even if you are paid to do it! When does for your own batteries, it tends to slip!
Your alternates to the time and effort can be as simple as making sure ALL drains are disconnected, then watch the voltage monthly or often enough to keep the best charge and good water levels. This often means adding an extra disconnect switch in the negative line to cut the drains left by the normal disconnect. A big surprise to many is that the battery disconnect DOES NOT disconnect the safety items like propane and CO detecotrs but leaves them to drain the batteries if we are not watching!
Or if we live in cold country, it may mean you need to take the batteries out and store them at the house where it stays warm and you can watch them closer.
Lots of personal decisions and choices to make to get the most out of them.
I might guess the better for you would be to let them charge, then totally disconnect the negaive lead, either by switch installed or by removing the cable. That will let you then connect the rV to power the humidifier without letting the batteries need quite so much watching for overcharge or running dry.
But that doesn't mean you are home free on work and watching as it is best to keep them charged at the correct level and once a month may be needed to do that untill you find what schedule fits for you, your batteries, charging sytem, and weather where they are stored.
For battery disconnect, these are some I have used as a simple way to avoid working the cables every time:
https://www.amazon.com/CZC-AUTO-Disc...A29D5JAUP1YACR
https://www.harborfreight.com/batter...tch-63425.html
Have you heard it said that the only thing on an Rv that works full time is the owner?
Linear Mode
