If you are going to add supply ships (like tankers) to the mix, then you have to add in the calculations and micromanagement of logistics and the resupply of fleets. A supply ship, if you were to have it, would only allow cruises for a limited amount of time before the fleet would either have to return to base (like a starbase, maybe) or have another couple of supply ships sent out to it (escorted, of course). What it can't be is an ersatz starbase you haul around with you to indefinately extend your range.
(And frankly, such ships would/should be a primary target - even worthy of sacrificing ships to kill it

).
While this is A scheme to handle logistics, it isn't the scheme the designer, developers and testers came up with. Adding logistics ships (IMO) is counter to the apparent philosophy of logistics implemented.