I have two lights on a panel inside the bathroom, a yellow and a red.. Dealer told me yellow means black water tank is 1/2 full and red means it's full, don't even THINK about flushing again.. We have now overnighted twice and the wife is constantly using the head.. I rigged up my own pump out system with a 12v jabsco mascerator and the appropriate hoses and fittings, I pump out into a 25 gallon honey wagon and that is easily dumped since we have septic at home instead of public sewer.. Nowhere can I find the capacity of the blackwater tank on the boat.. but when I pump it out, I'd say I'm getting a good 15 gallons or maybe even 20.. This is a jabsco raw water electric flush head.. Can I just flush fresh water down it on dry land to see if the yellow light comes on?.. Is it POSSIBLE I have HUGE holding tank of 30-40 gallons?.. This is a 2007 Glastron GS-259.. I don't want to flush fresh water and get a big surprise when I find out the tank level lights are NOT working...