I don't think the one Bruce found will work (at least on the outlets on our Axis) because they are snap-in outlets with a rectangular face. It appears that the snap power units have two "fingers" (technical term) that reach back and contact the screws where power connects. On the outlets on our camper those are hidden behind the rectangular cover.
Based on a different discussion here I picked up a couple of these guys:
Dual USB Charger Socket
These have the advantage of being 12V powered--you don't need to run the genny to keep them powered.
I haven't installed them yet as they just arrived yesterday.
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