Originally Posted by Gary RVRoamer
Typical would be a pair of 900-1000 CCA 12v batteries, which would likely be the Group 31 case size. Some coaches of that vintage used the somewhat larger 4D size, rated around 1200 CCA.
If you don't ever expect to be starting the engine at sub-freezing temperatures, you can save some $$ by buying a lower CCA rating (CCA is measured at 0 F). Plain CA, sometimes called MCA (Marine Cranking Amps), is measured at 32 F. That's usually more than sufficient for a motorhome battery.
Gary's advice is sound.
When our MH was new to us and our first diesel, the engine start batteries died when boondocking. With more experience I would have know that they were not cranking up to par. At the same the voltage regulator went out on our generator but fortuately has a old portable generator to fully charge the relatively new 4 GC2 house batteries. Used the 'boost switch' to start the engine.
When we finally got to Costco it was close to closing time and I got two smaller start batteries than was installed because that is what they had.
These batteries are now 4 years old. While I try to avoid cold weather, at one stop it was about 20 degrees F when I started a cold engine with out plugging the engine heater in.
In summary, you need two large start batteries for the diesel. My coach has 4 GC2 deep cycle batteries that can assist via the 'boost switch' assuming they are not delpleted from camping.