I own 2 of these engine in agricultural equipment and the tractor will start below freezing, maybe down to 10 degrees or so and the combine will start down to around freezing unaided. These are both the early mechanical injection systems and neither units have any sort of glow plugs or manifold grid heaters on them. I own a farm equipment repair shop (for 40 years) and this engine is very popular in the ag market and we have worked on a lot of them and they usually start pretty well in colder temps.
What year is this motor home? There could be 3 different types of fuel systems used depending on the HP rating and the year built. The early ones had a mechanical fuel injection system, then for a while Cummins put the CAPS (Cummins Accumulator Pump System) and then they went to the HPCR (high pressure common rail) system. The CAPS system was not very reliable and would not tolerate any air getting into the fuel system. The HPCR is a good system, but can be expensive to repair, especially if you need to replace the electronic controlled injectors.
Does it smoke out of the exhaust when you are cranking it trying to start it? Smoke would indicate that it is getting fuel, no smoke = no fuel. It may be something as simple as a check valve allowing the fuel to bleed back when the engine is shut down for a while.
As another poster stated if it is the mechanical system it could be something as simple as the fuel solenoid not pulling the pump linkage into the run position.
When I first bought my combine it would not start whenever it sat overnight and it was just a bad lift pump allowing the fuel do drain back to the tank. Very simple and cheap to replace.
