RV Repair: Waste water slide valve replacement., model questions, strap wrench

Question
I'm replacing a black water gate valve.  I need to move the flange fitting toward the black water tank in order to gain clearance to slide the old valve out.  The flange fitting pipe slides over the pipe attached to the black water tank.  They seem to have a sealant (similar to plumbers putty between them).

The problem is that I can't seem to loosen the flange pipe from the tank pipe.  Should I apply heat?, Cold? Strap wrench?  I tryed driving wedges between the flange and the valve with no luck, and stopped for fear of breaking something.

2004 sunseeker 31' class C motor home.
Please advise,
Wally

Answer
I ask that people not ask "specific model questions" for just this reason.  Different manufacturers do things different ways and unless I have actually owned the particular model and have done what is asked, I have not got a clue.
I would suggest you contact the dealer's service department as they are probably familiar with this procedure on your particular model.