How to find RPG games in my steam library?

First use your browser, not the Steam client, to log into the Steam Store. Then use this Steam Store search to find all the games that are tagged with RPG, Turn-Based, and Retro. All the games that are in your library will be marked with blue boxes like in the following picture:

Now in the Steam client you can set an appropriate category or categories for each of these games like Ragnagord suggested. You'll have to this manually, but you can put the Steam client and your browser side by side to make this easier. You also won't have to do all 800+ games at once.

An alternative method is to use depressurizer for Steam. It's an open source application that reads your game library and then will set the categories as they appear in the marketplace. As the blurb says:

Depressurizer can also autocategorize your games for you. Currently, it does so based on data from that games' Steam store pages. It can use genres, store categories (like "Single-Player" and "Steam Cloud") or tags.

Using these tags, you can set your game library into categories and easily find the game styles you want. This does rely on marketplace genres or user suggested tags, so it's not infallible.

I'm not associated with the program, but have used it before several times

You can set categories for each game (right click -> set categories...). You can set multiple categories per game. Also, searching for category names in the search bar will show all games with that category.

(I should note that i am using the newest beta version of the client, which may have new features implemented which might be unavailable in the regular client.)