If there is water in the gray tank, that "should" drain out. There are no other levers or valves to open other then sliding dump valve, one for gray water, one for black water.
The black water tank may have a clog in it. If the prior owner did not do the proper tank maintenance and fill the tank with some starter water before using the toilet, all the solids clump up and will not float.
You stated you could see water 10" down. If this is an actual 10", that is very far down. The floor is only approx 2" thick. That leaves 6" of air in the tank before the water. If the camper is on an upward tilt away from the dump hole in the tank, water "might" stay trapped inside on the up hill side.
You can try pouring 5 gallons of water from a bucket in the tanks and see what happens. If you have a clog on the black tank, see this post for some help. Start from here and read to the end.
http://www.sunlineclub.com/forums/f7...tml#post135567
If the gray tank is plugged, that is odd but not impossible. If you fill the gray tank from the shower, water will back up in the shower when the system is totally full. I would try 5 gallons and see if it comes out, if not then this one may take some Happy Camper Extreme cleaner or Tank Tech's to break it down in "time". It might take a week or more. I would really have to think hard on using any of the commercial acid cleaners on an RV tank. They could/can erode the tank probes inside the tank and may eat into the seals of the dump valve.
Hope this helps. Let us know how it goes.
John
__________________
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
Google Custom Search For Sunline Owners Club