NO expert but I would install a mechanical guage to double check the electric dash guage before starting to look for any other problems.
Electric guages, I assume your guage is electric????, are good but they depend on the sender and good electrical connections, all of them. Sender feed wire, + & - power leads must all be good for the guage to do its job.
Install a guage of about 100psi where the electric sender is located and see what it says.
If it says other than the electric guage then you may have a wiring or sender problem.
I'm also going to suggest that you need to get a Cummins owners manual. It should give an acceptable range of pressure at cruising speed and at hot idle.
--Are you using the correct oil?
--Is the level correct? Actually on these engines that is more important than many realize and is often misunderstood.
-- If you raise the rpm does the pressure climb? To what?
At 1200 revs the engine may not actually be producing its upper oil pressure limit. I know my engine [different] does not reach full high oil pressure untill around 1,500. You could simply be running it on the slow side for full pressure. I would be more concerned with the idle pressure.
I suggest also reading the following link. GO further as Seaboard Marine has offered a huge amount of free info for your engine.
Oil Pan Capacities for Cummins Marine Engines - Seaboard Marine