As far as the black tank you're probably going to have to cut the vent pipe. First drain the tank really well.
Go up in the roof and remove the protective cap over the pipe. Make sure the pipe is loose where it goes through the roof.
Remove the toilet and pull the screws out of the toilet flange. You will probably have to unscrew the flange from the top of the tank. On mine I used a big punch and a hammer to start the flange unscrewing.
Now go under the trailer and disconnect the sensor wires. Disconnect the drain pipe from the valve. Get a helper or a floor jack and a square of plywood to hold up the tank while you remove the brackets.
Drop the tank down and disconnect or cut the vent pipe. Make sure you leave enough pipe on both sides to allow you to glue a coupling back in when you reinstall.
If you want to remove the vent pipe you're going to have to find a way to push it up and remove through the roof.