Bring 2 extension cords for the campground 20 amp duplex would be the easiest solution if 20 amps is enough juice for your party.
If not, it would be very easy to wire up a box to do what you want.
The 50 amp plug is just two 110VAC circuits on one plug. The four pins are Earth Ground, Neutral (white), and two hot wires.
Make up 2 pigtails of 10 gauge (30 amp) exterior rubber coated 2 wire/w ground (BLACK, WHITE, GREEN) with a 20 amp Duplex socket in a waterproof box on each end. Connect one black wire to each of the hots, both white wires to the neutral, and both green wires to the "G"
However you MUST be concerned about overloading the wires and female connectors, since the outlet will deliver 50 amps to EACH of your 20 amp circuits.
A larger waterproof Pelican box could be used to make a "mini-fuse box" for the adapter. A nice sized one could be built to even hold the duplex outlets in the lid. Make sure you use waterproof covers on the outlets.
Look for two inline fuse holders/fuses rated at 20 AMPS VAC
. Get 3 waterproof wire clamps large enough to hold the rubber wire you are using.
Drill the ends of the box for the waterproof wire clamps and put everything together. The inline fuses go on each HOT leg into the box. Seal all openings with RTV or Silicone Caulk.
Carry spare inline fuses. You will most likely need them.