Well.. it took the better part of a day as we were pretty cautious to not damage the shell or frame, but we got it done. I think the shell is more structurally strong than we gave it credit for, but I'd rather take my time on this end and do less fibreglass repair if things went sideways.
The door was already removed, so I attached some wood from the outside to give it some additional rigidity.
We jacked up the frame slightly and removed the wheels so we needed less clearance. We built supports to evenly distribute the force along the outside of the shell as we jacked it up, leaving an exit route for the frame as best we could. Once the shell was high enough, we lowered the frame support jacks and muscled it out from underneath.
Pucker factor about 7 at this point, but once the frame was out, we could build up better supports with lateral strength so we're confident leaving it in place while we touch up the frame. It's braced into the fence beside in the interim.
Our holding tank was previously broken and I didn't realize the plug/valve (?) Was glued attaching the tank to the shell (hidden by poor patch job), so when we separated the frame from the shell, we made a small hole in the shell.. should be an easy fix.