Not even a drizzle =====> cable which operates the valve might have come loose from the valve. Yep, happened to me.
Getting at least a trickle =====> maybe a scrap of plastic/wood finally floated down and plugged the valve.
Another possibility ====> gray tank is empty or you're pulling the wrong valve.
That's all I've got without seeing your unit.
FR Wildcat 295RSX / GMC Sierra
Nights Camped: '13 = 49/'14 = 74/'15 = 74/'16 = 85/'17 = 110/'18 = 111/'19 = 86/'20 =108/'21 = 115/'22 = 40; Booked for 2022 = 70