Stuck (full) Grey Tank

[Rangoon]

Mrs. "Rangoon" here.
On my second gray tank in the back of the RV, the valve appears to be stuck tight - possibly rusted? I discovered this because the water is backing up into my kitchen sink and I'm unable to get the gray tank open to drain. I am parked in my backyard and not able to move the RV for several months. I called a helper but he could not do it either and was afraid he would break it if he pulled too hard. He said the underbelly looks too fragile to remove in this freezing weather, and even if he did remove it he would not know how to repair the valve or put the underbelly back together.
Does anyone have any suggestions for me or YouTube videos that show how to deal with this? At least for getting the water out? Possibly siphon it???
My helper said I'm going to have to wait till spring and just leave all that water in the sink but I don't think that's a good idea at all! I watched a YouTube video that said I could pour cooking oil down it if it was empty, but nothing about what to do when it's full.
(Background: this RV was my husband's toy -2016 Bunkhouse- but he died suddenly leaving it to me to deal with, and it sat in winterized storage for about a year+ until I could get someone to move it for me. I'm staying in it while building a new house, but I know nothing about the mechanics of it and I don't have a manual. So far I've had some very good advice from you people and REALLY appreciate this forum)!