I did it on my '73, which is probably exactly the same as your 74. The deteriorated plastic-sunlight-thingamajig was only on the forward vent; the other 2 are solid. there is a factory fan in this forward vent, as well, which really makes it impossible to get at the vent from inside the trailer, so I did it from the roof. But you only have to get up there twice; once to take it off, and once to put it back on!
I replaced the center piece w/ a piece of aluminum that I bought from HD. cut it to fit..taped it to the vent, then flipped it over and used the old rivet holes to make marks with the new ones. then I drilled them out, smeared a goodly bead of parbond around the outside, and used probably 2 dozen olympic rivets to fasten it together. seems like overkill, but I had to fill those holes in with something.