you can use any GC2 or T105 golf cart battery (2 of them). Note that the one Costco sells is the budget low end Interstate and has about 10% less capacity than the premium Interstate. Trojan makes the best GC batts.
Now that all said, I've got the cheap Costco batts in my E21, with the solar system and no inverter, we just don't use that much DC even when dry camping for a week or two. I have a little 300W inverter I use to run laptop chargers and such, usually during the day when I have peak solar, I find it rare when I've used even 10% of my batteries capacity
2014 Escape 21 (home away from home)
2002 Ford F250 7.3 Diesel 4x4 (tug)
1993 Mercedes 300CE Cabriolet (daily driver)
2006 Yellow Lab (Maggie... RIP Feb 2019