Remove Heater

[ChevyDan]

Hello, I have a 1984 Sunline C-801 truck camper that I need to remove the furnace (16M) to do some repairs under it where a big mice nest was, I can only slide the furnace about a half inch off from the outside exhaust port, then it stops, something is holding it from sliding off from it, was wondering if someone out there knows what I need to do to remove it, Thanks, Dan

[ChevyDan]

Was able to work around the heater since I read on another site that these heaters like to rust together inside because of condensation.