Jim,

Tom is right. We have also been to a number CC rallies both when we had a CC and with the Marathon. During a 5 day rally, you shouldn't come close to filling you black tank. Remember, yellow is mellow and brown flush her down.
We lasted 9 days last week at the Barrett/Jackson Auto auction in Scottsdale staying at the WestWorld campground. They have water and power. We were only at 45% of 68 gallon cap. on the black tank. We were 92% of 86 gallon on the grey. We ran the dishwasher drawer 3X's and took squirt and soap and squirt style showers. We opted out of having the sewer sucker. Upon leaving, we did dump the grey tank. Today, weather permitting, I will take the coach over to our local campground and dump it all.
Did notice a little trick that Newell owner was using at the auction. They had a black plastic tub to pour their grey water in for the sewer sucker to suck out, alleviating having the sewer sucker connected directly to the coach.