Hmm maybe i have C rating wrong for how it relates to trailer batteries. Ive always encountered C rating more as a measurement of performance. Not capacity.
Lets say you have a 10ah battery with a safe discharge rate of 2c. That means the battery manufacture has told the consumer they can safely draw 20amp from that 10ah battery without excessive voltage sag or excessive heat. Its not talking about the size of the fuel talk or how long you can draw 20amp for. Its more a measurement of performance.
That would obviously be the maximum discharge and charge rates. You could also use C rating to describe any type of charge or discharge rate. That same 10ah battery charging at 0.5C would be getting a 5a charge.