I have been running Costco batteries for 15 years and they have always served very well. First set lasted 9 years so still using the second set. I don't believe the quality is any lower. However they are not the maximum capacity available in the GC2 format. Costco of course does not make batteries. They are not really marked but I believe they are 208 AH. 220 to 232 is available if you really need the capacity. However in your post it does not seem capacity is a huge issue.
Lots of deep cycle batteries get a bad wrap due to poor usage, maintenance and charging habits.
I am not convinced you even need batteries depending on what you find with the converter. Better charging will make a world of difference with your existing batteries.
