Throughout the book, Greg's mom is always willing to help Greg. She tries to make Greg feel happy by making the trip more exciting. Greg's mom is a good companion to Greg. Greg's mom helps Greg for one reason only- because she wants to help him enjoy the road trip.
Rodrick on the other hand tries to make everything not so exciting. Rodrick will lend a hand if there is something in it for him, for instance when Greg was at the waterpark and he was running up the slide's staircase.