I found the exact Scamp
plastic frame replacement escape
hatch at Tweetys.com for $56. All holes were an exact match. After removing the trim and vent I found several cracks in the frame itself. I think these were caused by flexing of the roof by the weight
of the A/C and the 20+ year old plastic in the vent frame. To help support the new vent I cut a 3/4 inch plywood ring 1/2 larger than the trim ring and used stainless screws when installing the new vent. I looked at other options of support but plywood proved to provide the best support. One major problem was the corner screws in the vent frame and trim ring. The biggest problem was cutting down the trim ring depth by about 1/4". Over all it was not a major project but it helps to have two stepladders leaned againt the trailer on each side and you can reach all the fastners.