When I was 16 or 17, I got a root canal procedure done on tooth 28. I had no dental insurance and my parents paid for it and I got it filled. For some reason, the dentist said that a crown was unnecessary at the time. I was still young and I wasn't positive how the whole root canal thing works and it may have been due to my parent's inability to afford the crown, but I'm not sure. After the procedure I began taking really good care of my teeth because I never wanted that to happen again.

Now at 23, I feel like I may have a cavity in the same tooth because I didn't get a crown. Although I should've prioritized it, I haven't been able to go to a dentist because my parents divorced and could not financially support me and I've been working my butt off through college. I cannot completely confirm that my cavity is there having not been to a dentist, but if I were to get dental insurance now would it cover a new root canal and crown on the same tooth or would it be classified as a preexisting condition?

I guess I kind of have an answer to my own question. Typically there is a "basic services" and "major services" waiting period for dental insurance. This means that if you get dental insurance now it'll start covering preventative care immediately, however, you have to wait for several months before you can get something such as a filling or root canal done so that they can ensure that your teeth were fine until after you got the insurance. For example, the dental insurance plans that I was offered from United Health One require me to wait 6 months for "basic services" such as cavity fillings and tooth extractions, and 12 months for "major services" such as root canals and bridges.

Any experts, please feel free to correct me if I'm wrong.

Insurance companies will usually cover treatment on previously treated tooth as long as there has been a certain number of years between treatments, usually 5. Insurance may not cover the replacement of teeth if the teeth were already missing before the coverage came in to place.
Shop carefully for your insurance plan and compare plans closely. You don't want to be paying premiums for coverage you may never need.

Never believe that if a tooth has a crown on it that it won't get a cavity because that is not the case. Teeth with crowns can decay under the crown and teeth that have had root canals and then crowns can decay as well. The unfortunate thing about this situation (root canal and crown) is that often there is no sensitivity notice by the client and the decay can be quite extensive before it is found leading to replacement of the crown or removal of the tooth.

You may be able to have a dentist place a temporary filling to buy you some time until your insurance will cover the placement of the filling.