Is prepwork a real word? What is the proper form if so?

I have not been able to find "prepwork" written as a single word in any dictionary I checked (and I checked a number of dictionaries, including the OED). However, several dictionaries list "prep" as an adjective:

adj.
Preparatory: a college prep course; did extensive prep work for the interview.
_{American Heritage® Dictionary of the English Language, Fifth Edition. (Via The Free Dictionary)}

So, if we go by what the dictionary says, "prep work" (with a space) is correct.

Looking at usage, some people will write it instead as one word, probably by analogy with other words, such as homework and artwork. Here are some examples:

• Garden 2015: Prepwork!
• I started Vloging with AppAcademy’s prepwork, but Inspired by Freecodecamp
• Real Food Challenge Prepwork Day 5 – Mainport CrossFit

(It's also worth noting that there is a "personal research assistant" called PrepWork, which had some hits when I searched. If it's CamelCased, it probably refers to this.)