It "appears" my problem was the tanks. I cleanded the black tanks throughly last night. Today after driving 2 hours we stopped for a rest. I left the engine running....no sulphur smell from exhaust. Opened front storage area where battery was stored...no smell. Opened the cabin door and for the first time in about 5 days no smell. Drove for another 5 hours and no smell. To me that indicates the smell was indeed from the tanks but I don't understand what caused it to start doing it when it did. I guess that is a question for another day. Many thanks to all who had suggestions, any one of which could have been the issue.