I don't think I've ever heard "in a road trip" before (in the UK) except in certain uses. It may perhaps depend a bit on the verb before etc. For example I see on Google, "buy and sell antiques in a road trip around Cheshire" which sounds natural, with all parts of the road-trip being the purpose of the trip (rather than a road trip where you're just interested in the destination).
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AnonymousJust an example question,"A good map is your best friend in road trips" or it should be "A good map is your best friend on road trips"?"On".