Air Carrier First Officer reported a ground conflict during taxi to gate at TPA airport.

Narrative

The weather at TPA on the arrival was 1/4 SM and FG with RVR values shifting between 300 and 1200. We had to hold on the localizer at LAGOO until RVR values improved to above our required minimums; and we began calculating and refining Divert BINGO fuel and coordinating with dispatch to develop a plan if RVR values remained below minimums. After approximately 5 minutes in holding; ATC announced RVR values above our minimums and we commenced the ILS CAT III auto land to Runway 1L. After landing Tower instructed us to exit the W5 high speed and turn left onto Taxiway Whiskey. They also advised us to report leaving the runway. There was an Air Carrier on final approach behind us on the same approach. As we were rolling out on W5 I noticed a dark silhouette of an aircraft that appeared to be moving directly at us. It was extremely difficult to see; but I yelled 'STOP' to the Captain; 'the aircraft is going to hit us.' Because of the dense fog; the aircraft was still hard to see but it continued moving towards us from Taxiway V4. We applied maximum brakes; and the Captain moved to the left edge of W5 to avoid the collision. The other aircraft never slowed down; and if we would have noticed it a second later we would have collided. There was a second aircraft following the first; and it did not slow down either; and it passed our wingtips within ft. We immediately told Tower we still weren't clear of the runway on Taxiway W5; advised them of the near ground collisions; and continued taxing as instructed on Taxiway W. The remainder of the low visibility taxi to our gate went without incident. After we were safely parked and shutdown at the gate; the Captain called the ATC facility; and asked what happened. They stated that the two aircraft that almost hit us were 'not supposed to be there.'