Battery replacement time

My batteries have finally died on my weekend warrior FS3000. It currently has 2 Napa 12v grp24s. Hooked to solar charger and a onan5500.

I'm considering either going to grp27 12v or making the switch to 2 (for now, eventually 4) 6v GC2 batteries.

I'm looking for opinions or recommendations for both, which would you do and why?

Thank you.