We have a very small travel trailer. Believe it or not, my kids sleep in the bathtub for the first two years. After cleaning them up from the dirty day at the campground, I dry out the tub and place a folded quilt in the bottom. They can't roll out. It is a small tub, so it only works for 2 years. Oh - but turn off the water pump so your kid can't accidentally turn on the water at night! (yes, learned that from experience)
A bed rail probably won't work because the mattress is too thin to hold it in place. My father-in-law made a rail for one of our beds that attached to the wall, but walls are also thin in RVs and not designed to hold much weight.
Don't know how large the RV will be. Ours has absolutely NO floor space once beds are made out -except a small walking path. But my in-laws have a "bedroom". There is a small path on either side of the bed, my kids love to put a sleeping bag down there and sleep - they just have to remember not to get out of bed on that side. Some RVs would actually have space to roll out a small air mattress or sleeping bad, but many would not.
Overall, I would accept that you son probably won't sleep as well in an RV - different rountines, different bed, everyone in the same place - but those great outdoor memories will make it worth being a little sleep deprived.