Welcome to our club and sorry you are here.
I looked at proton beam therapy (PBRT) when I was first diagnosed. I have BCBS of Alabama and they first told me that they considered PBRT to be experimental, then from another online resource I found BCBS in Texas and Florida had approved PBRT so on my second round I was told by BCBS AL that they only would cover PBRT in states where there was a proton cancer center otherwise it was considered out of network. With the 2 closest centers to me being in Jacksonville, Florida and Houston TX I was SOL in Alabama. I gave up at that point and the fact I would have to move to a PBRT center and cover my living expenses for 6 to 8 weeks during the daily treatments just was too much anyway. I also ran into the same problems when I looked at brachytherapy as there were no really good PC experienced radiation oncologists in AL for brachytherapy and BCBS was reluctant to let me go out of state to Atlanta where there is an excellent brachytherapy oncologist center that specialises in PC.
I hope you have better luck in Tenn and keep us informed as to your journey.