Fresh water problem.

I have a 2008 Devon.  The fresh water screen is showing Water low and I have a full tank.   I have used Elsan tank cleaner and citric  acid crystals this year. At first it registered 75% then reduced to 25% now low.   Does anyone know if the gauge can be easily accessed for cleaning if the fresh water tank is lowered   and will I have any problems doing this.      Thank you

I cant believe no one knows how to drop the tank to clean the sensor. Please help.

Is there a large screw cap on the underside, often there is, which can be unscrewed and the sensors accessed from there?

Sorry constable, had not seen your post previously.
You do not need to drop the tank.
If the devon as I suspect is the same as most of the other models the water gauge works on a series of studs in the side of the tank one above the other and they make contact to give a reading when the water touches them. They are notorious for getting covered in limescale and gunge so then stop working properly. Different ideas and suggestions exist about using chemicals to clean them.

There is in my opinion a far better, simpler cost effective solution. 
Paul refers to a large cap in the tank. You can buy a hatch for retrofit, ours came from henderson tanks, the TCL4, they are relatively easy to fit for the competent DIY'er but should not cost much for dealer to fit one. Marquis fitted ours for free!!

The huge advantage is not only does this let you clean the points of the sensors easily you can also access the in tank pump if you have one so if that fails it can be replaced in a couple of minutes. You can clean and vent the tank.....you get the idea!!

Why all vans don't come with a hatch defeats me. It is a bit like the ridiculous way some models no longer have a door to access under the beds and the only way in is to remove the cushions....huh