When I get home I'll post a picture of the set-up I made. The trailer came with 2 batteries, and then I added 2 more on an isolator. The 2 additional batteries run 2 inverters, the Costco one like you have and a 2nd 800W that I installed in the storage compartment to run the TV.
Not quite as clean as yours due to smaller space and large battery cables, but functional.
I'm going to eventually get a solar panel to help with charging.
2005 Alfa See Ya 313 5er
2005 F350 PSD CC DRW Tow Boss