I can see getting one load of bad gas, but not repeatedly. Even stations that buy on the Spot Market don't normally get "bad" gas. The Spot Market is whoever has it the cheapest that day. The fuel may not have the additives that high quality gas has but it's not going to carbon and coke up an engine over night. It takes hundreds upon hundreds of hours for carbon to build up.
Have you sent a sample of the oil to have it tested? Usually if there is something wrong with the engine or fuel, it will show up in the oil.