If the new FF flange totally covers the old holes left by removing the old vent, then IMHO, you don't need to use an aluminum piece to cover it. When I install a roof fan, I use a liberal bedding of SikaFlex 221 under the flange, bed it into the SikaFlex, attach the flange (I use screws) and then apply a very liberal dose of Dicor self-levelling roof sealant or more Sika-Flex.
There are several methods and choices of sealants and butyl tape works also. I prefer the SikaFlex for it's bullet-proof sealing capabilities. It is one tough sealant!
Just be sure that you aren't going to want to remove or re-position this thing while you own the trailer.....'cause it ain't gonna move!