A friend of mine has a givi tank bag with the tank filler ring connector. He now carries an allen key to fit the tank ring as two or three times he has been unable to open the filler cap, he has to do a bit of adjusting then it pops open.I've used the Givi tank bag, with the ring on the tank cap, with zero issues for nearly 4 years. Never heard of anyone having issues with them damaging the tank..sounds like your dealer is just trying to sell you a BMW bag
BMW large tank bag for me. It lives on the bike as it carries all the boy scout stuff, 7 years and 60,000mls, still completely waterproof.