We were also experiencing similar gray tank drainage issues last winter. Our tank acted as if there was a "dam" across it and no water would flow out the drain unless it was over the level of the dam. As a result the tank never really drained and we managed OK simply because we usually had full hookups.
This spring we had it power cleaned in Kerrville TX by a company franchised by All Pro Water Flow
It was pretty amazing what they got out of the tank! Ping pong ball-sized pieces of calcified "stuff" came out; lime-based deposits like one finds in a boiler system. It took an hour to "scrape" this crud off the walls of the gray tank. The black tank was clean by comparison. After it was all done, water flows easily out of the tank and the gauge reads normally. It was definitely worth the $200 I paid for the service.