It sounds like they were below the interstate and hit the interstate bridge above to me. Most gasoline tankers in the U.S. drive local. They would know if the interstate clearance was too low well in advance. Most interstate bridges are at 13'9" or above and yes a tanker is lower than a freight trailer. This is just my guess but it sounds like they were trying to take a short cut or were headed to a delivery point. It sounds like they were on a secondary road neither one knew. Not clearing an interstate bridge on the interstate with a tanker doesn't make sense period.
I don’t think the accident was caused by a bridge clearance issue. I do think it was caused by a nut (or two) behind the wheel.