Welcome to the forum!
I’m a little surprised you can’t camp for 4 to 6 days with the 4 6v batteries you have now. I can camp for 3 or 4 days, with four kids, and heavy use, on 2 6v batteries. How long are those batteries lasting you?
How are you determining the batteries state of charge (how much energy you have used)? Are you using a battery monitor or checking the voltage?
Sorry if you know this already...if you discharge flooded cell batteries below 50%, and especially if you are running them down to zero all of the time, you will damage the batteries and they will no longer hold a charge. Also, the cells need to be topped off with distilled water about once a month.
The on board charger on your trailer should be fine unless you change to lithium batteries.
Here is a video on my off grid set up. I use the “keep it simple” philosophy...