(I also have the Hepvo trap with 1 1/2" 'shoe' on it that fits this perfect..

There is only about 1 1/2" of threading length on it.. and the tub has runners molded in for drain angle and support..
The drain threads are just under 2" and I tested a 2 1/8" hole saw and that gives me a bit of wiggle room before the threads would hit my 3/4" Drymax floor..
The question is.. do I thread the drain to the tub only?? or the tub and the sub-floor as 1 item using the drain to hold it all together??
All the pics online I've found show the drain attached to the tub/showerpan and just a larger hole thru the subfloor and usually outside under the RV is the drain threads and swivel P-trap adapter..
The tub recommends keeping the HOLE small under the tub to try and support the drain area and said something about plumbers putty..??