Join Date: Jan 2015
Location: West Palm Beach, FL
You really need solid backer behind the drywall. I'm still a fan of plywood and a backer bolted into the studs.
In lieu of that, lag screws into the studs on the bottom of the cabinet and toggle bolts through the studs at the top of the cabinet. The bottom will have more shear action on the fasteners while the top of the cabinet will try to pull away from the wall - pivoting on the bottom fasteners. The negative with drywall behind the cabinet in this scenario is drywall will dent and compress leaving more room for play. With play comes fatigue and with fatigue comes failure.
And after you do it, make sure you're comfortable sitting on it. As shoot summ eluded to, it will be used for more than brushing teeth. If you're not comfortable sitting on it, it won't hold up in a rental setting.