Not long ago Mike Desch had us scratching our heads over a battery that seemed to check out OK, but wouldn't start the engine. That problem was eventually traced to a bad battery cable. Because it's time for my bi-annual smog test I decided I'd better make sure everything was in working order, including the batteries.
Even though we hadn't used the MH in almost a year I keep it connected to shore power with the coach and engine batteries shunted, so they're all kept fully charged (plus, it's my man cave!). To my surprise, after disconnecting from shore power and removing the shunt the engine battery (2 years old) wouldn't turn the engine over. If I pushed the Auxiliary Battery (emergency start) switch, which connects all the batteries together, the engine started normally. Suspecting a bad battery, I checked the voltage and it registered a normal fully charged 12.75 volts. Remembering Mike's problem, I then used an ohmmeter to check the continunity of the cables from battery post to their termination points, but everything checked out normal (continuity and low resistance).
Prior to removing the battery, which is a dirty job, I decided it would be a good time to top off the water on the coach batteries. The water level was surprisingly low, so I then decided to check the engine battery and 4 of the 6 cells were so low that I couldn't detect any liquid. Filling the cells and letting the engine charge the battery for a few minutes solved the problem. The lesson learned, aside from the need to regularly check the cells, is that just because a battery has plenty of voltage doesn't mean it can produce sufficient amps to turn an engine, and water is a lot cheaper than a replacement. RVing is a learning experience. Good luck. - Dave