One good theme could be their family life. How they raise Marron and deal with the daily challenges of being parents in the Dragon Ball world.
Another place to check could be dedicated Dragon Ball fan forums. There are often threads where fans share their own fanfics or recommend ones they've read. Some of these forums might have a specific section for family - centered fanfics which could be where you'd find Android 18, Krillin and Marron stories. You might also try social media groups dedicated to Dragon Ball. Fans sometimes post links to their fanfics there.
There are definitely some out there. However, the quality can vary a great deal. Some might focus on their friendship, while others might explore more of a bromance - type relationship. It really depends on what you're looking for in a fanfic.
The fanfiction could explore their relationship growth over time. How they gradually became closer and more in - sync with each other. It could show how they supported each other through tough times, like when Krillin was worried about protecting the Earth from new threats and Android 18 was there to give him confidence and strength.