Gray water tank

I don't need a gauge for the fresh water tank, because I can look at it.

Has anyone installed a guage for the grey water tank, or is there any way to add an additional tank for more grey water capacity? One frustration was never quite knowing how much water was in the the grey water system.

Along the same lines, is there anyway to increase the capacity of the black water system?

I've a 1997 2619.

We just got back from a week of dry camping at a National Forest Campground. We'll be doing more dry camping and I'm planning on how to add more capacity for the fresh water system. Adding more capacity for the black and grey water would be nice.