Hi @lca_inc and welcome to the iNat Forum! This is definitely doable, but requires some hands-on manipulation of the URL in your web browser. There is a detailed tutorial here on how this works. The search parameters of particular interest to you can be found in the Other observation properties section of the tutorial.
A relatively simple search that may come close to what you are looking for would look like this (click on it!):
From there, you can get more nuanced about exactly where you search for the term “roadkill”. The above search looks for the term “roadkill” in four different places in an observation: description, tags, names of taxa, and place descriptions. There are additional URL terms that will restrict the search to a single one of those areas. You can also search for user-created fields related to roadkill.
See if this helps, and feel free to post further questions here.