There is no evidence, other than anecdotal, that Irish Spring will keep mice away. There’s a video of an RV’er who tried the Irish SPring method to keep mice ou tof his old camper and they actually chewed on the soap. The video. Feel free to give it a shot, but I’d recommend two things instead:
1. Stick with proven repellents like peppermint oil, chili oil, and live traps to remove any mice in your camper.
2. Most importantly, the most effective form of pest control is removing entry points so they can’t get in, and then using a rodent repellent as a last line of defense.
I cover exactly how to do that here: https://camperfaqs.com/how-to-keep-mice-out-of-camper-best-methods
Hope this helps!