Cummins engine barely starts

So per your other post you were looking for a fuel shutoff solenoid and a starter solenoid.
What was the problem and which part fixed it?

Still not knowing if you mean low cranking rpms or takes longer to fire but appears to crank fast enough, I'll go with low cranking rpms.

The solenoid is basically an externally mounted relay and generally they work or they don't. A "delay" wouldn't cause low cranking rpms (from low voltage or other causes (like worn out starter motor or low cranking capacity of batteries or bad cables/connections).
It an on/off switch that triggers the power to the starter motor. could be a bad connection or burnt contacts in the solenoid though. Although the contacts in the bendix would be the ones carrying the high current from the batteries to the starter motor.

Unless hard to access in your chassis, the solenoid is a cheap part and easily replaced without removing the starter. If you can physically reach it and have even a basic set of hand tools and understand righty tighty lefty loosey, it's a 1/2 a beer job.

Thanks to all for your help.A diesel mechanic came to the unit this morning said the batteries are sufficient although old. He recommends cleaning the terminals he thinks the starter solonoid is going out because there's a slight hesitation when engaging the starter. I have a call into Cummins to see if they have a solenoid or starter so I can have the whole package put on there I appreciate everybody's help.

OP has stopped providing diagnostic breadcrumbs, so time to move on apparently.
Sounds like it’s a P pump ISC, so about as simple as they get and bulletproof engine (in a RV anyways) with as low of miles/hours on it.
Fuel, air and good power to the starter is all she needs.

Phoenix? Batteries don't last long in 120* temperatures.

Original equipment. 1998. 126,000 miles. You have been very helpful.

To better describe, differentiate “kicking over slow” from “cranking slow.”
One says low voltage, or bad batteries or bad cables or possibly starter motor on its way out.
The other says it takes a long time to get fuel.

No fuel vs cranking too slow are most likely totally different issues.

How old is the starter?

Before spending $$$ on someone else, check both positive and ground connections between chassis battery and engine.

Also, while someone else is cranking the engine, monitor chassis battery voltage at the battery and then at the starter with your DIGITAL VOLTMETER.