If you use the fridge for hours at home and don't notice the smell ; then my best guess would be that during the trip the sink " P " trap is draining enough to allow grey tank odor through ( grey tank with food particles can smell as bad as the black tank) and putting the dawn into the trap re seals it.
Next trip , put a ziplock bag with water in it over the sink drain to see if the odor stays out of the interior.
Secondly , and perhaps more important , go to the lower fridge vent on the outside of your RV and remove the vent , and closely examine the area for any yellowish powder around the chimney where the propane flame is positioned .
If you see yellow powder SHUT THE FRIDGE OFF ! the cooling unit is leaking and will need to be replaced. Both Dometic and Norcold fridges can leak and when they do they're a fire hazard.
EDIT: Also check your propane leak detector for battery and look for the best before date , replace as required.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.