there are several things you could do, install a vent, unless you are doing it for the room to install a celing panel, you could put a chunk of wood and more of the same roofing material on top and seal with eternabond tape so it is almost flush, on the inside you can go to windsor plywood and buy the same interior panels as the rv industry uses, cut a small square, insulate the hole with foam block insulation and then put a patch panel on with trim around the edges to make it look good. is it ever going to look as good as if it was never there, nope but you can do a pretty good job making it look acceptable if your mind is to it.
there are several reasons to do this, the extra room for panels, like has been said, mabe just to get a more efficient and quieter AC, or maybe to lower the overall hight of the unit. the last two you could just install a fantastic fan in its place and it would look like it came that way from the factory. or maybe if you use a rack system to mount your panels instead of closing off the hole a fan would work there also.
I think I would seal the outside hole with an aluminum plate and closed cell foam on the underside if I planned to put solar over the top. On the inside, I would probably finish a wooden plate with a cluster of recessed puck or eyeball lights for task or mood lighting.
yup, everyone probably has their own ways, the reson I would use wood is thats what the rest of the foor is, so if you have 1/2" plywood for the underlay, I would put that, and since you have access to the inside of the celing where the vent is you can block it and make it the strongest section of the roof. we are both on the same page for the closed cell and a light would be an idea but are you going to have a recess once you get the same amunt of foam in there as the rest of the trailer.. I supose you could as the insulation might not run the full thickness.
I am tall which is why I suggested recessed lighting. The new LED fixtures are thinner, so there are more options for recessed or surface mounts. You could even use ribbon or rope lights if maximum insulation is the priority.
So, to answer the question about what I did with the hole. On the roof with the opening I cleaned everything up then put a layer of double sided tape down then a solar panel on top. On the inside, i added a piece of plywood to give support just in case anyone stepped on to the panel on the roof and not realized there was a hole there. For the future, I will be adding a "ceiling fan" based off of 120mm computer case fans and a fan controller. I had thought about lighting here as well but focused just on the fans. These fans can be very quite yet move a decent amount of air. My goal was to have some air circulation inside the camper.
Here are some images to clarify things.
