2024 Tab320S Boondock

On a 2024 trailer, and all the trailers since about 2019, there are Victron solar controllers. The solar controller has an app that monitors the solar input and provides battery voltage and basic charging status.  It isn't perfect, but battery voltage (if you read it correctly) can give a decent approximation of battery state of charge, even of lithium.

As Hammick said, there are various "packages" that may include the Victron shunt, which is a nice step up in battery monitoring, since it can measure used amp hours drawn from the battery.

The "old" type battery monitors, such as the KIB monitor: they really did not do a good job of giving a readout of a lithium battery.  They were built to only read a lead acid battery, and, since lithium battery state of charge readings are dramatically different than lead acid, the KIB was simply not worth using.  The Victron equipment, or the other various brands of "shunts" do such a better job. Even a multimeter (properly used) would work better.

For lead acid batteries, the battery monitors inside the trailer also had limitations, since the battery switch had to be on for the monitors to function.  Consequently: they always gave a misleading idea of the actual battery status.

So, tell us what "battery package" you are thinking of having in your trailer, and we can show you what these nice Victron readouts look like.