Adding coolant to diesel

Start with the correct questions:

How old is the present coolant-- "regular low silicate for diesel with added SCA" is generally good for 3 years with SCA added when it tests below 1200 PPM (test strips).

OAT'based coolants basically good for 6 years with no testing or adding unless you drive over 300K miles.

So, if you don't know what coolant is in there or its age, REPLACE IT-- lots cheaper than a new radiator or engine!

And, yes, I recommend the OAT coolants. If Caterpillar, Cat ELC. If Cummins Fleetguard ES Complet OAT.

I would use a quart of NAPA NowCool to up your SCA level if you have not checked it's % with a new test strip. Use your antifreeze to fill the rest of the way. Could not find a listing for it so something like this.

You should check with a Cummins diesel mechanic as to what you should put in your engine.

What does the level in 'coolant reservoir' indicate when rig is level?

We have been parked on a pad with the front lower than the back by about a foot, wasn't sure if that would give a false reading.

Yes, you can add directly to the "Cummins" reservoir - that's how your supposed to add. (preferably when cold)
But more importantly you should find out where the coolant is going.