Charging the trailer battery while towing is iffy at best. The voltage drop through the wiring is such that not much current gets to the trailer batteries. If you have lithium's the stock charge wire is not going to help at all. Did you get the lithium upgrade?
Even "way back in the old days (2017)" the Classic was a bit of a burden on the stock lead acid batteries. Solar plus AGM's quickly became the way to go. I would *guess* you have solar. Is this correct?
Charging lead acid batteries to "full" can take a long time. The only way to be sure they are topped off is to run on shore power for 24 hours. Yes, that's quite a wile. You should be able to charge in 8 hours or so. It does depend a bit on how low your batteries are.
Lead acid's run from around 12.6V (full) down to 12V
(stop using / half full) at normal temperatures. If it's really hot or very cold those voltages will shift by as much as half a volt.
Battery disconnects / flashing lights / beeping propane detectors normally are going to do their thing down below 11V. Your batteries are well past the stop using point by then. This is not good for the batteries.
Quick hints for battery use:
1) Never run the inverter. If you do, run it briefly and then shut it off.
2) You have a use/store switch and a full disconnect. Understand how each one works and what they do. Based on other threads running the full disconnect on "manual" may be a good idea.
3) If you have a compressor fridge, it pulls a bit of power. That plus the normal drain of all the Classic stuff can get you up to a fairly substantial drain even with nothing else turned on. You very much want to turn off lights, fans, .... when they aren't in use.
4) Unless you are on shore power or a generator, put away the hair dryer, forget about the electric coffee machine, leave the crock pot on the floor of the wardrobe. Running them on the stock lead acid batteries .... nope.
5) Watching TV on battery power is tempting. That inverter is gobbling power while you do. The TV it's self gobbles power. A couple DVD's in the evening is probably more than the battery will be happy with ( on top of all the other stuff).
Yes, this list goes on and on and on ....
1) Do you have solar? If so is it factory or dealer installed?
2) Do you have the lithium upgrade?
3) What do your battery voltages look like?