Adding an external battery

I have a 32’ Travel Trailer with 200 watt solar on roof for dedicated DC frig. . Just changed to 2 new 100 amp AGM sealed batteries. I have a third, was going to go the inverter route but have changed my mind, just gotten the AC200L. The little I know about the “whole trailer power thing” the Bluetti seemed simpler than the inverter. So, my question is, if I get the D40 can I use the extra AGM sealed battery as an extension for the AC200L, just plugging in the 30 amp plug into my shore power trailer connection and turn off my converter breaker in the trailer box. I still won’t know how it works, just want to know if it will…
Thanks for any help…

Can you use a 100Ah AGM to boost capacity of an AC200L = Yes. However, It needs to connect to the DC input of the 200L and will be a slow trickle charge, likely not keeping up with the 200L demand loads. Also, once the AGM drops below a certain voltage, it won’t charge the 200L. You really need to get a 12V to 24V booster to be effective. Last, an AGM will only give around 50% of it’s capacity i.e. 50Ah to maintain its cycle life. A far better method is to use a LiFePo4 100Ah as the voltage is higher and you can use most of its capacity. There are plenty of Youtube vids available online concerning the subject of expanding power station capacity.

@GraveDigger You can connect the 30A output port of the AC200L to the 30A shore power interface of the trailer. Yes, be sure to turn off the converter circuit breaker to avoid conflicts between the two input sources.

Do you mean that you want to use the AGM battery to extend the AC200L for charging it? Where do you want to connect the D40? Could you provide a diagram draft if convenient?