I don't know why they have snapped off but most likely they were over torqued at the factory. One way to remove and replae them is to cut a small slot in the protruding steel that is left and use a standard slot head screwdriver to remove. If the head has broken off flush with the body you can buy left fluted or reverse drill bits. While you are drilling out the screw usually the screw will start to unthread itself.
If you have aluminum siding on your TT you may want to leave the screws snug but not overly tight to allow for expansion and contraction.
Additionally, depending on what the screws held on, you can remove the rest, take the component off, and using a small vice grips remove the broken off ones. This then would be a good time to replace the screws/bolts/etc with stainless ones. Stainless is not that expensive and the appearance factor is way beyond the small cost.