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There are no dumb questions, and welcome to the Forums.

Tell us...what battery do you have?  I'm guessing a "stock" battery.  Open up your battery box in the front tub and have a look.  Most likely, it is a "12v, Group 24, 80 amp hour deep cycle battery".  This is/was the same battery many thousands of 320 owners had installed in their trailers.

What are you using to monitor your battery?  Your 2021 has solar, and a Victron solar controller.  When you say the "battery gets low", what are you using to determine that?  The Victron solar controller has a bluetooth app that will help you to monitor your battery.  You might be using a multimeter.  If you are using the Kib monitor in the trailer...that is ok, but not very accurate. 

Let us help you get the Victron app running on your phone.  If you don't have the passwords the original owner used in the App, some hoops will have to be jumped to get it reset for you. But, the Victron app, or, even a multimeter, used correctly, will help you know a lot about your battery status.

What you are seeing "overnight" as the battery "drops" is called "parasitic drain".  You guessed it: the LED's and What-Nots are what the parasitic drains are.  There are some devices in the trailer that are never completely off.  The carbon monoxide detector, for example, is always on.  These parasitic drains are not very much, but, over time, they can add up.  In my 2017, my trailer, just sitting there with the battery switch on, nothing running and no power coming into the trailer, adds up to about 1/4 of an amp per hour.  So, over 24 hours, about 6 "amp hours". 

This is what you are seeing overnight, or about those 12 hours when the solar is not charging the battery, until the sun hits the panel the next day.  If you have that 80 amp hour battery we mentioned....and your "parasitic drain" was say, 10 amp hours overnight to make the math easier...that would be about 10 percent of your total amp hours in your battery. (It is a bit more complicated than this, but you get the idea.)

You are correct that a healthy battery should hold a charge "with nothing turned on" for quite a long time.  But: unless the battery disconnect switch in the trailer is off...those parasitic drains mean the trailer is still "turned on".  If your solar did not get enough of a charge to replace the "parasitic drain" each day, the battery could eventually drop to a very low "state of charge" that can become problematic for the health of the battery, and things in the trailer may stop working on battery power.  (If you did not have solar, and left your battery switch on....the parasitic drains can take a "stock" battery lower than what is good for the battery in less than a week or so.)

Have a look at your battery and tell us what it is.  Most likely, it will have some sort of designation with the number "24" on it.  Do you know how old it is?  Think it might have been replaced with something larger?  Take a picture if you want.

Do you have a multimeter?  Found the Victron app?

Again: do not hesitate to ask questions!

Ok, good.  The Victron app should be your preferred voltage monitoring tool.  The trick about a lead acid battery is: if the battery is being charged...the only thing your tool will measure is the charging voltage going to the battery.  If the battery is being "discharged", even by the small amount of power used by these parasitic drains, the voltage readout is going to be automatically "false", and depending on what is being used by the battery, will always read lower than actual.

The only way to get a good readout of a lead acid battery is with the battery "rested":  No charging, and no power going out of the battery.   So: battery switch off, and in the Victron app settings, turn off the "Charging enabled" toggle for a few minutes.  Let the voltage settle a bit, and as it stabilizes it will settle down to a more "accurate" reading. 

There are many threads about owners using various switches to reduce the parasitic drain on the Jensen units, etc.