Hi, I'm a little late to the party and you may be back from camping.
I do not know your older HW heater. You said leak and then heater and I guessed it was the hot water heater and not the furnace.
I have not seen a HW heater with the vent riveted to the panel but on the real old ones maybe
I do know the putty tape can keep the unit stuck to the side of the camper pretty good. To the point it feels like you are going to rip something apart trying to get it out. If this is the case, try a warmed putty knife around the seal to the side of the camper to help break free the seal.
Hope this helps
If you still having issues, post a pic of what you are up against and we may be able to help suggest some ideas.
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
Google Custom Search For Sunline Owners Club