Is there any way to calibrate the oil pressure and water temp gauge to what the engine is actually doing? Mine always show low and I know they aren't. The gas reads fine.

i read in a FSM one time how to do that, but the FSM was for a car.
i think erenburger did an article in mopar action magazine on this as well.
if i remember correctly, there are tiny screws you use to tweak the reading.
also, if the voltage limiter isn't working correctly, that will show a low reading.
the gas gauge isn't connected to that part of the cluster, so that's why it works as it does.

How much is the error? These systems (gauge/sender) are far from precise, even when brand new.

Both read less than a 1/4 of the scale and I have checked with a direct reading gauge and the temp and pressure are fine.

Since the fuel is OK,it's most likely faulty sensors. If you momentarily ground the wire at the sender, does the gauge go full scale?

Could it be a faulty voltage limiter?

i was wondering about the voltage limiter as well.
if it had just been the water temperature gauge, i have seen the typical greenish/gray, or grayish/green deposits on the sender throw the temperature to the coolish side.
when a guy polishes the sender to a bright brass finish like it was when new, the gauge returns to normal readings.
as this tarnishing occurs over time, it takes awhile for it to actually affect the gauge reading.
it seems it takes a "just enough" amount of this deposit to affect the reading, otherwise, it reads as it should, until the "just enough" tarnishing amount of deposit is reached.
as to the oil pressure gauge reading low, i have seen small pieces of engine gunk get into the sender, causing the gauge to read low.
as these are impossible to clean out, a new sender is the only way to fix this issue.
however, all new senders are not created equal, so when a guy replaces the sender, all i can say is "buyer beware".
now with BOTH gauges reading low, it almost points to the voltage limiter as the problem like stumpy says, but before a guy rushes out to buy one of those, check the limiter's connections to be sure they are clean and tight, and also check the dash grounds to be sure of the same.
just my experience and opinion over a 55+ year span in life working on this brand of toys, and experiencing the same issues.
your mileage will vary.

Myself, I'm leaning towards the voltage limiter, cause the oil sending unit is a new Mopar unit. Maybe, I'll try a new one from R/T electrical.

Fuel gauge IS run off of the voltage limiter with the temp and oil gauges. Amp gauge is not.
Do you have the original sending units or replacement ones? A lot of replacement ones are not accurate. There is company making accurate sending units but can't think of their name right now.

truck was like this when I bought it. I put a new Mopar oil sender in it, seemed like it worked for a little while, then went back down. I checked with a mechanical gauge and the engine has great pressure.