to buy or to rent for a road trip.

Hi and welcome to the forum. And, we welcome you to the US. From your original post you wrote that you have not RVed before. I humbly suggest that when you arrive here and pick up your unit, you go directly to a near-by campground and begin to get familiar with all of the everyday aspects of RVing. This could take a few days to a week. You will need to learn to safely back-up, pull into gas stations (adapt to rear-end swing), hook-up electric safely, dump your black and gray tanks, level or block the RV to make it level in the campground and other RV chores. But don't fret about this. Most RVers are friendly and will be glad to assist you with any of these tasks.
Remember to bring some favorite toys or games for your son in case you get caught in some really rainy weather and need to stay in the RV for a day or so. You might want to bring a few DVDs or buy some here for you and the misses for the same reason. Also, bring or buy some rain gear for all. You don't want to stay in the RV every time it rains, too much to see and do. Finally, as your plans come together, give us some idea where you might travel so we can provide some hints. For example, if you plan to go to Yellowstone NP, bring some warmer clothes. It can and has snowed there every month of the year. We have experienced snow and cold there in June several times. Best wishes for a great trip. Bob