OK, we think that it is NOT exhaust. After much crawling around and pulling panels, it appears to be road grime with a hefty amount of oil/grease, etc. The buildup on hauling makes sense as well. I am now satisfied that we will survive our travels. I will get the dealer to replace a few of the plastic plugs running along the bottom of the vehicle which might be allowing rainy globs from entering the ventilation system. And, our tailpipe is remarkably clean so we don’t think it is getting sucked back into the panels that way either. Thanks to all for your prompt suggestions.
Rod