I am going to assume (possible mistake) that your tank gauge is reading full when the monitor panel is reading 3/4.
Something to keep in mind is a propane tank can only be filled to 80 percent capacity to allow for expansion. The tank gauges on my coach and my prior Class C were calibrated to read full when the tank was filled to 80 percent. With that said, your inside panel panel may not be calibrated to take the 20 percent factor into consideration.
My prior Class C had a procedure for calibrating the monitor panel indicators but I haven't seen that procedure for this coach, and in all honesty, I haven't thought about it until I read this thread. I do believe, however, the procedure will be very similar.
Here is a link to a video and info on how to calibrate the monitors at the panel. It is being done on a Newmar but again the procedure should be about the same.
For LPG the calibration should be done immediately following a tank fill up.
RV Geeks Videos: How To Calibrate RV Tank Monitors
Hope this helps.