My little brother used to work at a Long John Silvers.
He could not even wash his work clothes with other laundry; they were so greasy.
He confirmed that yes, they fry fish, chicken, fries, etc. all in the same grease. Which is why it is hard to tell the difference between their fish and/or chicken.
Where I worked all we had was fries back then, but when the breakfast crowd got heavy, we’d cook the BACON in the deep fryer. The fries tasted a lot better after breakfast!..................