Normally the grey tank is fairly accurate in my RV. The black tank is never very good in any RV, as a little TP and it can short all the electrodes inside, showing water is covering all of them. When there is liquid that conducts electric from the base ground probe to the second one up, it shows 1/4, the next probe 1/2, the 4th probe 3/4, while the top one shows full, even though my probe is about 2" from the top of the tank. So I have a little wiggle room.
The problem might be that once the tank is 100% full, some can drain up into the shower pan, and that can smell. Or even when 90% full, and driving can lead to some showing up in the shower pan. I have a tub/bath, so even if 10 gallons shows up there, I will not overflow onto the floor. Be careful doing dishes, as the water will go down the higher elevation sink, and show up in the shower pan.
Many times to save grey tank space, I would use a dishpan for dishes, then toss that water onto some nearby thirsty plants or the grass, much like all the tent campers might do. Also taking a shower, I can plug the tub drain, then use the dishpan to put extra water into the toilet, I have a 59 gallon black tank, and 42 grey. Not all the water, but say more than 50% will go into the toilet.
Also to save both fresh and grey water space, I can put 1 gallon or so of dish or shower water into a used 1 gallon jug, and use that for flushing at night.
I can camp solo for about 3 weeks without overfilling the grey tank, and a couple of months while living on a ranch without draining the black tank. Many times I was dry camping in areas where the bushes needed some water, so I would dig a hole, and drain the grey down into a 15" deep pit, then bury it by night, so no bug problems.
Fred.
