When I first started doing plumbing they didn't have the closet bolts that are used today. We used a brass 3" 1/4"-20 hanger bolt with wood thread on one end and machine thread on the other. With these, you can drill through the flange and into the wood substrate with a smaller bit and hold the toilet down with both the flange and the wood underneath. Line your holes up, drill with undersized bit, install hanger bolts by using cap nuts on the machine end. May have to hold bolts with vice grips to remove cap nuts.
When I had mentioned replacing the flange and tube,,, I have ONLY come across one that was actually glued,,, ( Its wasnt glued from factory,, it was obvious the previous owner glued it by the residue he left behind in his attempt to seal it ), The others were threaded or slip sleeved with a rubber vibration gasket,
John T did make an excellent point tho that there are RV's out there that have the tube glued directly onto the holding tank.. My 5th wheel is an 85, (i.e. 30 yrs old) and its a slip sleeve design with the rubber vibration damper gasket at the bottom... Looks like a rubber coupling but no clamps and takes lots of soap for lubricant to get it to slide together. I think it was designed this way so holding tank can be dropped easier if need be and not have to worry about snapping off a glued tube.
I have always wondered if you can get the wood screw ended johnny bolts for a case like this and just drill a pilot hole thru the plastic.
I had thought maybe I could cut notches into the existing flange, but how to you keep the bolts from turning? I rolled the dice on the $50 kit. It should look nice and clean with the base that comes with it.
The new toilet comes with a gasket. It should seal up to the adapter kit. I need to bring the floor up to the height of the flange first since it's screwed down through the carpet. I think it will all come together okay.