if you join Air tanker, you join as a civilian flying civvy registered aircraft. There is an RAF side that fly the voyager (military A330), there is an option to become a sponsored reservist in which case you would fly the voyager and the civvy 330s. However with that, you would not do any air to air refuelling or tactical stuff. It would be purely RAF passenger transport albeit with slightly different procedures to the Civvy Jet.
If you don't become a reservist you will only fly the 5 civilian A330's of which one is permanently servicing the south Atlantic air bridge, 2 are damp leased to Thomas cook flying long haul, one is about to go to Jet2 and the last one is on reserve doing ad hoc charter flying for whoever wants it.
Overall, good mix of flying which would be even more so if you opted to go down the reserve route.