I wouldn't recommend Marine Tex for around leaking rivets because it is an epoxy and will set up hard (and then probably crack). Not that epoxy is not wonderful stuff because it is, in the correct applications.
I'm normally not a fan of adding sealant on top of something that's leaking because it doesn't ultimately work and just makes more of a mess to deal with when you/someone eventually does fix the problem; however the rivets that Scamp/etc. use do have a hole in the middle, and the only way to really seal it is to add sealant to it (Scamp now uses rivet caps along with the sealant, but you have to put those in when you install the rivet). So if it's simply the hole in the middle of the rivet that's leaking, you might be able to add some sealant there.
Otherwise, I agree with Donna. It's time to pull them and replace them. You could use the same type of rivets again, or you could go with closed end rivets (they don't have the hole). You can also replace them with machine screws, washers, and nuts. Rivets are great --- especially when you can't get to the backside (why they are "blind") or when you have to build 1,000 trailers economically (because they are quick), but they are only one of a number of options that can make sense when you are replacing a few yourself, on one trailer, and can reach the back side.
On the vent I would also remove and re-seal, as others have mentioned. Depending on the condition of it, it might make sense to get a new one to install; they are generally not all that expensive, and also not "lifetime" durable.
Also as Donna mentioned, I would not accept any leaks
as long term "I can live with its." It's too easy for water to slip behind things and cause rot, rust, or mildew. The shell of our eggs is fiberglass, but there are a number of wooden parts (floors, furniture supports, etc.).
Nice to have a new egg in spring