I have a 40' long one for my boat and camper. Wish it was 50. The back of the boat sticks out the back of the carport around 6 feet in the winter with a tarp coming down off the wakeboard tower.
I keep the camper in front of the boat mostly and like having the nose of the camper under the carport at least a foot or two. Reversed, you can stick the back of the boat under the nose of the camper but it is difficult to access the rear of the boat because the camper will be lowered down on the jacks for stability.
Started with a short bed camper and a 21' boat like yours. Now have a long bed camper and a 24' boat...
I found my carport on Craigslist for $1K, but had to take it down from some guys place, haul it to mine, and put it back up. Had it for 4 years now and no problems