Cummins engine barely starts

Years ago we had an issue with a fuel cut off solonoid for our Cummins engine model 6C 8.3. It would it be difficult to start and I replaced the solonoid and it would start right away. now when we got to Fort Worth the engine would not start and barely kick over. Cranking very slow.. after 2 hours we got it to start and I'm wondering if it's just a bad engine battery. I did couple the coach batteries together with the engine battery and it's still kicked over very slow . We have a mobile mechanic coming tomorrow and I'm wondering if it sounds just like a bad engine battery. Voltage reads 12.8 in the coach but that could be a a surface charge. Any thoughts? It's been hard to start for a while, never kicking over fast. I hope it's not a sticky cutoff solonoid because there are none left in the US!