Hope you spoke to the owner, not just a salesman or manager. Unless they have already filed the paperwork with the state, the camper can still be considered new.
While they are unlikely to let you back out of the deal, they should be willing to let you negotiate another deal on a smaller unit.
However, keeping the trailer you want and buying a 3/4 or 1 ton truck to safely pull it will make you happier in the long run.
And no you didn't "Mess Up". You caught the mistake before you wrecked the rig. So mark it as a "good save" in life's memoirs.
Happy Camping.