With only a 20 gallon fresh water tank and no way to easily tell how much water is left (unless one crawls under the camper), I install this nice gauge. Bought it from a company in Australia named RV Electronics for under $60.

CLICK A THUMBNAIL TO VIEW THE FULL SIZE PICTURE. YOU CAN SCROLL THROUGH THE FULL SIZED PICTURES BY MOVING YOUR CURSOR TO THE LEFT OR RIGHT EDGE NEAR THE CENTER AND PRESSING THE  ARROW, OR TAPPING THE EDGE (MOBILE DEVICES).