Why not remove the existing drywall, then replace? I cannot comprehend how the job would be easier or faster to just drywall over the existing drywall, since that has ramifications for the door jambs, window jambs (if windows), all the electrical outlets would need to have those little extension pieces, etc. Crown molding or any other molding would need to be trimmed to fit, etc. Is the toilet roughed in far enough from the wall - do you have a 1/2" gap between the back of the tank and the wall? You still have just as much mud work to do, so why not gut and replace?
And, given that bathrooms are prone to lots of humidity, it's likely the wallpaper was applied with a paste that doesn't come off very easily, and/or the drywall wasn't primed prior to putting on the wall paper. So, I do agree, often times it is much quicker and easier to just gut and put up new drywall, vs. attempting to remove very stubborn wall paper. Materials are cheap - no more than $200 in materials for a typical sized bathroom, green board (or purple these days) being only a tad more less cheap than regular drywall.