If you're looking for a single 12 volt deep cycle battery. Interstate also manufactures that style.
https://www.interstatebatteries.com/...s/gc12-hcl-utl
It's the GC12 and is available at Costco as well as "other fine outlets" throughout the US. Here's the Costco link:
https://www.costco.com/Interstate-12...100476402.html Our Costco has them in stock nearly all the time, but I understand that some stores don't carry them and have to special order for you.
Notice the price is about $50 more than two GC6 batteries, the available 20 amp capacity is very similar and the 12 volt battery weighs 85 pounds. That 85 pounds makes it difficult for most to "wrestle on the tongue or in the battery compartment" of most trailers. That's why most use GC6 rather than GC12.
But, Interstate does have 12 volt "true deep cycle batteries" and the link above is just one of many places to get one....