Looking at the post
OK My Sprinter 05 model now done 38 thousand K's about not far off 20 thousand miles
Interior of the exhaust pipe is a charcoal Grey [Dry powder form in looks]
Out side of the exhaust pipe same shade graduates up the outside pipe about 1/4 of an inch at Max. same texture no extended ridge from carbon build up.
The Carbon soot is dry and very smooth in texture, similar to graphite
It is not abrasive. to feel.
If the soot is sharp and it can cut your fingers if it's severe, it will also have an oily sticky texture, half way between dry and wet.
Under the exhausts surrounding areas, there should be very little smoke or soot evidence, the most that would be visibly noticeable will be on the side of the draw bar, it will have a candle like flair in appearance, but also very little build up of carbon soot. More of a distinct shadow line, but an evidence of light soot will be there.
Back door fumes can accumulate after a two thousand mile trip, or if allowed to build over a period of one to two weeks, If you have evidence of clear black ink like spots that are sticky to touch, then this is the high sulfur content of carbon crystalline particles that are unburnt from the engine Its fuel and oil related. same as LSD Fuels
The main exhaust collector in the states could be different as to the Euro ones, the length of the exhaust is two feet By 12' wide by 7' high, its huge, if you a smaller unit on the Dodge then this unit will release more particles into the rear exhaust pipe, that will end up on the rear doors, very noticeable if it has dust on it as well from a gravel road.
I took three pictures just on sunset they are out of focus, because I did not have the light to set the focus on micro zoom, but clear enough for an indication I can run up perfect shots tomorrow if you wanted a clear shot.
IF the tip of the exhaust has sharp and a some what sticky grease build up in a Crystal like formation on the out side of the exhaust, with the inner section of the exhaust pie that contains an indicator of dampness, then you have to change the oil and filters and look for a higher quality fuel ULSD and test it to see if it is actually ULSD fuel.
Richard