We have also noticed this "burnt rubber smell" in our 2005 40FDTS. It has been difficult to track down since it is intermittent but can be quite a strong smell. It seems to be coming from the dash A/C vents and mostly from the ones that blow air on the windshield.
We have gone through all sorts of interesting gymnastics while driving along setting the A/C switches to different things and then climbing over the dash and under the steering wheel (that one gets interesting) to sniff out where it might be coming from.
The exterior air intake is on the firewall, passenger side, behind the gen slide, but we can't smell anything there when we stop to fuel and don't see anything in that vicinity that could be causing the smell.
Sometimes you can smell it slightly even when the A/C or heater is not on. Sometimes it crops up when climbing a big hill - maybe, not sure. Otherwise, no clue. But it is worrisome that a fire will result or a hose blow out somewhere.