Where the heck is the fuel gauge calibration screw???????

Look for the sender on top of the tank...usually passenger side, but varies by coach. About hockey puck size with four wires. Directions below are from the Monaco manual. VERY SMALL adjustments. Good Luck :)

Fuel Sender
The "Centroid" fuel sender has no moving parts and works by
measuring capacitance (electrical property) between inner and
outer tubes in the tank. The more fuel between the tubes, the higher
the reading. Electronics in the "hockey-puck" head of the sender
convert the capacitance to current to drive the fuel gauge.
Connections:
The "Centroid" sender has four connections:
1 & 2 - Positive (POS) and Negative (NEG): Battery voltage
to run the electronics in the sender head.
3 - SEND: Connects to the SEND terminal of the gauge on the
dash.
4 - ALARM: Makes a connection internally to the negative
(NEG) terminal when the low fuel alarm level is reached
(when the fuel gauge is reading about 1/8 tank). This turns on
the fuel indicator light on the dash and is not adjustable.
Adjustments:
The "Centroid" sender has two adjustments:
1 - EMPTY: Adjusts for length of sender. It has been set at the factory, covered with a
sealant and should not be changed.
2 - Full Adjustment (FULL): The full adjustment can be used to correct for slight differences
between fuel meters. During installation, it has been factory calibrated and should
not need re-adjustment.
The correct adjustment technique, with a full tank of fuel, is to start with the full adjustment screw
completely clockwise. This should cause the reading to be above full. Adjust slowly, rotate counterclockwise,
until the full mark on the gauge is reached. The intent is to always adjust downscale rather
than upscale.

BTW Each diesel engine with have 2 hoses to the tank top, perhaps near the sending unit.

A common Monaco setup has the Centroid sending unit outside the rails on one side, look for wires on the top of the tank. There are 2 adjusting screws and instructions on their website.

The only way I have ever been able to "calibrate" a vehicle fuel gauge is to remove the sending unit, bend the float arm, and reinstall the unit. Repeat as necessary until the desired calibration is achieved.
The sending unit will very likely be in the top of the tank, approximately in the middle. Since the manufacturers usually are not smart enough to build an access panel in the floor, the only means of access is to lower the tank.
Good luck.