If all gauges are faulty it suggests fault at control panel. Check if there is any electrical current going to them, some gauges will go to full rather than empty when there is a fault with power supply.
As Opensauce says float gauges work on a variable resistance as the float moves an easy way to check is disconnect at the gauge and touch to Earth the gauge should move in open circuit it should move in the opposite direction, if there is no movement I would check to see that there is power getting to the gauge.
------------- Retired and living the dream touring Europe
On my motorhome there are no float gauges just a stainless steel screw that protrudes into the tank from the outside which is connected to the control panel.
when the water level reaches the screw it makes an electrical connection back to the control panel and an alarm sounds.
There are screws at different levels in the tank to show 1/4 1/2 full etc.
Take a look inside the fresh water tank ( there is usually an inspection screw top plug) and you can see if it works on the float gauge or screw method.
Basa
If all the water,waste are all 100% and no air temp then I think it may be the control unit
Is there any type of reset facility?
You could try to switch of all power to the control unit and see if it resets itself
Basa