black tank wont drain

[Ziphead2]

I have a 2017 Minnie Winnie and am having trouble with my black tank draining. I dont know if the issue is the gate valve outside that is stuck or what. I let it sit until; next stop about 150 miles. I thought whatever is stuck would work its way out. Before we left i put some chemicals down to include Happy Camper but still no go. I am thinking the gate valve is broken closed? If not I'll have to work my way from the valve to the tank. Any opinions? I am open to any ideas/as always thanks!

[jdc1]

Open the toilet and look inside. Is there any water? Add water and drive around, to mix it all up real good. If that doesn't work, it's gonna get real messy. Run something semi flexible up the waste connection. Better have a few huge buckets on hand, or be right next to a cleanout.