Canon supports the idea that Wheeler and Trish once dated. At the very least, their relationship goes back a long way and they were obviously very close before Wheeler left and became a Planeteer.
In the episode Talkin' Trash, Wheeler fails to recognise Trish when he first confronts her. She has stolen his Planeteer ring, and when he goes to get it back, he asks if he knows her. Her response to this is to knee him in the stomach, indicating personal scores and feelings.
Trish appears bitter towards Wheeler, and is obviously angry at him for leaving. It is strongly hinted that Wheeler has never contacted Trish since becoming a Planeteer.
They do, however, have a history, as Wheeler talks about how Trish used to come by his place for dinner. His mother also apparently has several of Trish's artworks hanging up in her house, indicating that Trish spent a lot of time there and got on well with Wheeler's mother. Trish also has significant knowledge of Wheeler's relationship with his father - at the end of the episode she has contacted Wheeler's father in an effort to help Wheeler bridge the gap.
Verminous Skumm refers to Trish as Wheeler's ex-girlfriend as he mutters to himself while putting the final stages of his plan in place.
Wheeler and Trish also walk hand-in-hand up the street after Wheeler rescues her from the speeding train, and they kiss after he pulls her from the water towards the end of the episode.