It would be helpful to understand more about what is going on so maybe others can offer advice. Reviewing your forum history of 12 posts I find nothing about your new-to-you trailer other than mention of Li battery in the previous post ....
- Is this trailer new direct from ETI will all original ETI components, or from a previous owner with perhaps other than ETI installed components?
- What is the battery (how many?), solar (how many panels?), and inverter configuration you have?
- What loads are you placing on the battery (what 12 volt devices are you using and how much / how long)? Are you running a compressor fridge? If not, are you running your fridge on propane? Are you running the furnace and if so, how much?
- Are you using an inverter to power 120VAC devices (microwave? TV? Etc?) and how much? When not actually using the inverter are you ensuring it is turned off?
- How are you charging the battery other than the (apparently non-functioning) DC-DC charger when driving (Solar? Any hookup to shore power?)
- If using solar, have you had good sun exposure and for how long each day? Are you charging using the solar array when driving?
Yes. lots of questions, but all go to understanding you battery depletion situation and trying to offer relevant advice rather than random guesses based on incorrect / inapplicable assumptions.
There are likely folks who can help you here, but they need to understand the context of the problem to do that in a meaningful way