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You can install additional batteries without the need for a new charge controller. The GoPower charge controller does not have a limit as to the number of batteries it can supply charge to. When adding in additional batteries be sure to check that the additional batteries have a minimum of 13V, they must also be wired in parallel.
Be sure that the batteries you install are the same supplier and part #. Ideally, they would be the same/similar age, but this is not a requirement.
Important note when adding additional batteries, as per GoPower specifications the maximum allowable battery bank size is 4x100Ah batteries. I have attached screenshots from the GoPower battery data sheet for your information.
We have the U-shaped dinette in our 5.0 with a 4 inch increase in depth on the “back wall” seating (standard is 18” and we have 22”) and it’s still tight. I’ll try to get a picture today with the seat hatches open.
Understood regarding the space limitations in the battery storage compartment with the standard dinette. We will have added adjacent storage compartments with the U-Shaped dinette option. We don't have a problem making an opening for the wiring & adding more battery boxes in the adjacent U shaped dinettee storage compartments.
We just picked up our 2021 5.0TA (August 13) with this option. Our trailer is currently in storage so I can't provide a picture but there are two separate LiON battery boxes, one for each battery, and there isn't much additional room for any additional batteries. Perhaps with some serious juggling and rewiring you could add a third battery but it would be challenging at the very least. This doesn't directly answer your question regarding the solar controller's ability to handle more batteries, but it may be a moot point if you can't actually add any additional batteries.