I see some on the P & S site with 5-20Rs. However, looking at the instructions for the Leviton brand GFCI I have in the house, it says they have models with "20a Feed-through rating, 15a outlet rating" and also models that are "20a Feed-through, 20a outlet" So I suppose this P & S one I have is marked 20a for its feed-through and it has the 15a outlets.
Anyway the circuit has 14 wire and a 15a breaker. American versions of this same trailer have 20a breakers instead, according to the owner's manual. Don't know if they have 12 wire and 20a outlets.
The Leviton also has the line terminals on the end where if "up" that would put the outlet ground prongs "up" To me that is "up-side down" :) I might have to add some wire to the short lengths showing in the box to fit on the receptacle if the top ones are supposed to go on the bottom and won't reach. All part of the fun.