As the spring for those original vents gets weaker with age it does not provide the tension that would keep the top tightly aligned. Think the hinge is a little sloppy and the weak hinge doesn't push things into place.
Butyl tape just works well and for a long, long time. Not adhesive so it can shift a bit under load AND is easy to clean off for replacement. Caulk not so much. Silly-Con can work when applied like a gasket to the parts before assembly but I don't think it is as long lasting or easily removed.
IF it would fit inside your storage clearances I would give some consideration to a fantastic fan. Low power draw 12 volt fan that can move enough air to make a difference in a small scamp
. And it has a vent cover so.... two birds thing. I know fantastic fan has a low profile, also you will want to contact the company and ask for their thin wall spacer piece. They make a piece for installing in our single layer FG campers which don't have the ceiling thickness of a stick built.
There is a 12 volt line running along the side of the window up to the overhead cupboards to the small 12 volt light
at the end. Should be able to splice into that line and run the short hop of wire from the cupboard to the vent in a slit in the ensolite. Just touch up with white caulk afterwards if slit is too obvious.
Don't think you will find a vent that fits the existing hole easily if you do please post the information. I think every post I can recall on replacing that vent involved a little bit of hole size increase. RV's are bigger now so vents are bigger?
Rivets vs. stainless steel bolts is an endless debate. I think direct replacement I would go with what has been there for a few decades, upgrade to a fantastic fan and I think you might end up using the fasteners commonly used with those which I doubt would be rivets. Plus 1 on small aluminum washers to spread load or shim but don't forget you can pull the pin from a rivet, cut down the tube, replace the pin and viola! shorter rivet that custom fits your wall thickness.