Group 27?
Those are RV/Marine batteries which are "combination" starting/deepcycle, they do either job poorly. Basically built for light duty trolling motor applications or in a pinch starting a small gas engine.
With a GC it won't take long to destroy the group 27 RV/marine batteries..
Many yrs ago, my Dad tried RV/Marine batteries in his truck for starting and to power the TC.. Didn't work well for starting his truck on cold winters days and marginally worked OK for the TC.
If you want your GC to work correctly, just go to Sam's or Costco and get their 6V or 8V GC2 batteries and be done with it those batteries are reasonably priced and work well.. No need to buy super expensive name brand GC batteries..