What to do if my fiance sold a car over a month ago but the buyer has yet to get the title in his name and is using toll roads racking up $300 so far?

Your fiancee needs to go your local county's tax office and fill out a form to show that she is no longer the owner of the vehicle.  This is called a vehicle transfer notification form.  Here is a link that will help you read up on the notice before she goes to the tax office:  https://vision21.txdmv.gov/Vehicle/MainTransferNotification.aspx    .  Once she fills this out, she can then send a copy to any agencies that may try to bill her for the toll bills.  The purpose of this form is for situations just like your fiancee's or someone who has a wreck in a car after it's sold-- it puts the world on notice that this is no longer your car and you're not going to be liable for any incidents involving the car. 

You also ask about being able to go get the car back.... you can't do that if the car has been paid for.  Regardless of the title, you know that the other person is the legal owner and taking the car would be considered theft.  Instead-- fill out the form described above.  You may also want to contact the buyer and push them to get their part of the transfer done.  It's possible that the seller needs to talk to your fiancee.  Many people don't realize that when an informal purchaser goes to switch the title that they also need an additional signature by the seller.  This is a relatively new requirement that has put a significant kink in many people getting their titles correctly changed over.