Winnebago cold air leaks

What I finally found was that the A/C-heater box is held to the front of the firewall with a number of 3/8 nuts. The box is plastic and has a rubber gasket attached to it that seals it to the firewall. Looking at the bottom of it from under the front passenger wheel well there appeared to be a gap between the gasket and firewall. I found a hacksaw blade easily slid in the space. Either the gasket had shrunk a little since the coach was built or the nuts holding the box to the firewall were never properly tightened. I suspect #2. I snugged up all the nuts and now do not have that gap. Waiting for another cold snap to see it that was all of it...