I understand the concept that if the furnace is heating the interior air there’s really no way of pressurizing the interior. You would need to solely pull from the outside to do that. I would imagine that the warm air escaping from the cabin ( through doors, windows, and vents) is actually sucking this cold air in through the underbelly.
So I climbed up under and scoped things out. I found one sizable hole where the gas lines go up to the water heater and furnace. Shooting a light up inside, the appeared to be sealed through the sub floor. I then went up stairs and looked under the kitchen sink where the water heater is and that air was the same as the interior. No draft. Therefore the kitchen must be sealed off pretty good. I stuffed some battened insulation up inside the hole underneath. Went around and stuffed insulation in the slide out shafts. The Chloroplast is really fairly tight and all the sewer drain penetrations are styrofoam sealed. I’ll plan on running a 97 foot bead of caulk around the outside edge of the chloroplast just in case but there’s no real gaps in it. The basement storage area is pretty tight as well. It has a 3” gap at the top of one side where is ties into the kitchen sub floor but no drafts. It is heated. Though it’s 2017, I don’t think this unit was pulled much and it’s shows like a brand new one. I feel a huge reduction in the draft after doing the little bit of insulation and the furnace didn’t seem to run as long. I did notice that the lower floor registers are pretty cold when the furnace is off and the draft is apparent through those. Removed some of the foam sheet insulation. I placed under the steps and some of the draft , no where as much as when I started, cane back and the draft in the registers significantly decreased.
I can tell a big difference in temp on the wall panels on the kitchen head wall. The lower portion ( furnace) is 30 degrees colder than the upper portion ( water heater). I’m thinking of taking the lower paneling off to access the furnace and see what it would take to seal that area from the underbelly cavity. Maybe install a filtered grill in the paneling.
I really appreciate everyone’s insight and input into this. I think we’re getting to the bottom of it.