Is there a real-world test of a battery's true capacity?

55 ah would be the 50% mark.. Also 12.1 volts that you drained it down too is the 50% mark.

This AH rating is based on a 20 AH rating..

55 AH / 20 = 2.75 amps.

That means that if you put a 2.75 amp load on your battery, then in 20 hrs your will reach 50%. That is if your batterys start at 100%.

5.5 amps will last 10 hours... ect..

But you need to check voltages as most batterys are rarely charges to 100% with the crappy convertors in the RV's.

Or check with hydrometer for best accuracy..