I'm assuming you swapped the wiring at the gauges or you would not be asking where the sending units are? I had a similar issue but swapped the wires at the sending units.
If your 2000 MADP is the same as my 2001 with the Mountain Master chassis, and I think it is, then the sending units are basically directly below the brake pedal on the OUTSIDE of the floorboard. The treadle valve for the brakes is located there.
Best access is to make sure you've got the RV secured with leveler jacks, jack stands and wheel chocks. Then crawl under the left front (below drivers seat) and basically sit up. Look up and you will see the steering box and directly above that the treadle valve with the sending units. While you're there, check the bolt on the steering shaft to steering box. Mine was only hand tight from the factory.
I can't believe it actually stayed on without falling off all these years. I only found that last year as that's when I bought the motorhome.
Forgot to add... I bought the new sending unit at finditparts.com. Cheapest price I found anywhere at about $65 if I recall. That might have been with an online coupon which isn't hard to find.