Hi Steve,
If there's no exterior access, you'll have to figure out how to remove the furnace. But I'm surprised as there pretty much has to be both an intake and exhaust vent on the exterior. That's where the access panel is typically located.
But maybe back up a bit. I'm also surprised that Heartland Customer Service was able to provide this specific guidance. They typically don't have that level of technical knowledge. So a review might be in order:
When the thermostat calls for heat, you may hear a "click" inside the coach which would be a relay at or near the air conditioner. The furnace blower will start running, pushing cold air for about 20-25 seconds. At that point, there should be 3 ignition attempts. I think you would be able to hear the ignition attempts if standing by the intake/exhaust vents.
If you have a sail switch failure, the blower runs and pushes a normal amount of air out the vents. But after 25 seconds, a failing sail switch prevents the ignition attempt.
So...if the blower is running for 25 seconds and pushing what seems like a normal amount of air, it could be the sail switch. But if you can hear the ignition attempt while standing next to the intake/exhaust vent, then it's not the sail switch.
If the blower doesn't run, it's not the sail switch.