I thought I had seen a ceiling support method here on the forum, where someone put a curved piece of material (metal or wood?), up one side of the trailer on the inside, (where it had an attachment to transfer any weight
from the top to the vertical support), it curved across the top,and down the other side. the radius of the inside piece was slightly greater than the curve of the top, forcing the top to go up and give the top great support. if the vertical pieces had a stop piece at the bottom, then they could transfer the pressure of the top being pushed up. the side pieces could be run from the floor up to the bottom of the curve of the top, then the top curved piece could be forced into place by a jack attached to a 2x4, backed by a curved template piece, allowing the jacks upward pressure to be equal along the top.