We buy used rv trailers and Unfortunately the damzed things are already installed in the trailer. All three rv trailers we've owned had at least one some where.
The first thing I did was buy a jug of white elastomeric product like Kool Seal and put several coats over the outside of them. Cuts the summer heat and not so noisy in a hard rain/hail storm.
I use ethafoam cut to fit snugly up inside and use a couple of strings like in the the pictures to hold it in place. However I didn't add strings to our current rv trailer and it falls out while in transit but stays in while in otherwise.
Maybe someday I'll have the money to order new a rv trailer but a skylight will not be on the roof. Too much heat loss in the winter and heat gain in the summer for my use.