What are the Causes of bad gas in gmc vans?

the biggest cause of bad gas is sitting idle for more than 2 months with whats in gas it only takes one month to start to degrade at 2 months of just sitting the gas has lost alot of octane and doesnt burn as well, the second is filling your van with a gas can this can cause 2 problems water and filter plugging dirt, if your suspecting water in the gas use "dri gas" sparingly as the ethanol that is now present in all gas is basically "dri gas" are you sure its a bad gas situation and not a bad sensor?