You might have to remove the petrol hose to give your self room at the rear of the filler. My gas filler has a flexible hose at the rear with a brass joiner to the filler bit. - 1/2 inch spanners (or around 19mm) where needed to undo.

The may be a little bit of gas in the line - but as the tank has a one way valve, you can crack it and let the little out.

Unless you have a wild open area way from all flame - I would not bother venting the tank.