[HoosierDaddy]

Hi Tim,
I tend to choose the path of least resistance. I'd put Black tank chemical in and let-r-set for a month or so. At the 2 week mark I'd add a surfactant...(plain ol dish soap or liquid car wash soap). Drive it on a bumpy road to "shake it up"
If that doesnt work its gonna get messy. I'd open the drain valve and try to force a water hose in. Hopefully the force of the water will soften the goo as it goes until you reach the liquid. Once it starts to flow it will probably break loose completely.
Last resort would be to disassemble the drain valve where it mounts to the tank and "mechanically" remove the goo.
Re: plan #2 . Any "plan" that uses air pressure sounds very "entertaining" for those of us who are observing!!!! B-) Seriously if it's solid enough that it wont drain I dont think that air will liquify it.
The air actuated valve is helpful when it is hooked up to a sewer drain at a campsite. It is usually best to hook everything up and keep the valve closed. Once the tank is at least 3/4 full you can activate the valve and dump. This keeps the solids moving with the liquids as it drains.
Good Luck!