at what's listed under "regional boards" and "ad boards".

Those would be your main concentration areas.

However, this board offers more than regional advertising, and a lot of hobbyists and providers travel, so there is value to be had no matter where you are.

Wisconsin, Ohio, Atlanta, Florida, Carolinas, Washington DC, New York, Boston... That's not an all inclusive list, but that is probably the biggest TER areas (though I may have missed some). So as you can see, it is not an east coast or west coast thing, or southern/northern thing as there are areas all over the country where TER is popular. However there are also a lot of areas where it isn't that popular. As you mentioned, Texas and Louisiana (there is another competing board more popular there). Also the northwest (Seattle, Portland) uses other boards more. While TER is big in souther CA, it is not as widely used in San Fransisco and the central valley. (again other boards dominate)

As Clarence said, look at the regional boards and regional ad boards, but I would ad, to look at which ones are the busy boards, and also look at the review cities and see which areas have the most reviews.

Also as Clarence mentioned, it is a good idea to maintain a presense on TER even if you aren't in a TER dominated market. It could help you if you tour to an area where TER is big, and may also help if a hobbyist from a TER dominated area visits your area and has no clue about the local boards.

No way to account for the non-posting lurkers.  Posters are easy to see.  Over all, more posters west of the Mississippi, I believe.  I would bet this site is most popular in the privacy of home or hotel room, but some do look at it in airports and airplanes.