Cat convertors can smell, but are usually worse , during a hard pull up hill when exhaust temps are at their highest, and the smell dissipates the longer your on the road.
Look at your tail pipe extension, it should be 2>3" past the side wall of the RV. Check the body seams above the exhaust system , for missing sealant; most manufacturers spray foam the seams then undercoat over the top to prevent exhaust fumes from entering.
Does your coach have a CO detector?
Is your propane leak detector a combination CO/propane?
Are you driving with the roof vents open ?
Does this smell get in while you're using the dash A/C ?
If you're traveling with the 1 lb. propane bottles in a compartment in the rear of the coach, try moving them to the toad to eliminate them as the cause.
Propane should always be transported in a vented area.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.