[JRTJH]

If I were trying to close off a 5" hole and mounting a 3" plug in its place, I think I'd fabricate a 1/8" thick 6x6 or 8x8 aluminum plate with the 3" hole cut in it. Paint it to match the remaining plastic/metal components on the trailer, then use butyl putty tape and put it in place, securing with 4, 6 or even 8 screws if needed. Then install the 30 amp receptacle to the hole in the plate.