Furnace exhaust is like people, it will take the path of least resistance. Check to make sure there is no restriction anywhere in the exhaust vent. You are correct about an improper air/gas mix causing sooting. The air intake for the burner may be blocked by dust, lint, spider webs and their ability to collect debris, pet & human hair, etc. Spider webs can do a serious job of blocking exhaust. If the burner has an adjustment such as a vane or machine screw in the air path then those typically must allow the maximum amount of air when burning propane
fuel. If you usually
camp at altitudes over 3000 Ft. then a service center or propane
dealer might change the gas orifice to restrict it so it will be balanced due to less air being available. Propane
appliances must be de-rated by 20% in my community because of altitude. Manufacturers of propane RV appliances have to choose a "happy medium" since one may be camping at sea level one day and 8000Ft. altitude or more the next. Ideally one would change appliance orifices to suit the altitude location, but in the "real" world no one will devote the time or energy when they can be outside enjoying themselves.
The moral of the story is make sure the burners and venting are kept clean. Whether you feel competent to do the job, or you have a propane dealer or RV service center either teach you or simply have them do it, it should done at least annually. If you camp or drive where it is exceptionally dirty or you notice black accumulating around the vent, then the task should be done more frequently.
Keep yourselves safe and...Happy Camping
& Ann K.
One other problem could result in black appearing as you describe. If the heat exchanger has failed and developed a hole. This can prove to be deadly and requires immediate attention before the appliance is used again! Turn off the propane supply immediately, until replacement has been accomplished.