Camping Trip was a great episode, I mean yeah River is crazy about camping, but 10 freaking bucks, is was because "Oh wait what? my precious flower of a daughter is going to be hanging out with her boyfriend all alone in the woods? No way! I'm putting a stop to this right now!"
You could tell just by his interactions throughout the whole thing "Piss off wanker, you ain't gonna be my son." Then realizing "Oh wow, he's actually a great guy from my daughter."
Then that semi coy "Accepting him as a future son in law" played out in Game of Flags, and the Toffee arc when River was even calling him "My boy, and leaving things to him."
It's one of the great parts of Star Vs, the unspoken but very clear bonding between River and Marco.