I have a 99 American Cruiser and as with yours, it has a combined black/grey water tank. On ours, there is only one set of probes on the front side of the tank near the middle of the van. They are wired to both the black and grey tank level indicators on the panel so both always read the same. The fact that you are getting different black and grey water reading indicates yours may be wired differently then mine. When we bought the unit 2 years ago, the level indicator panel always showed both tanks as 2/3 full so I put in a set of hooded Holst Miracle probes. Initially, they worked well, and they still show when the tank gets above 1/3 full up to full, but it never shows below 1/3 even after emptying the tank. You can see pictures of the mod here.