You could do it by installing a standard Marinco (ParkPower) 20 amp inlet like in the photo. The duplex recept. on a pedestal is usually 20 amps (and GFCI). Install a 15/20 amp T-slot receptacle in a convenient spot inside which might be close to the inlet to make running wiring easier. Use a 12 gauge (20 amp) extension cord. You would need to use a connector that fits into the Marinco inlet. I would buy a standard #12 cord and install a new connector like the Marinco one pictured. Marinco products tend to be rather pricey and you could either shop around the internet or use an equivalent brand.
You don't want to use a 14 gauge (15 amp) extension cord and recept. (inside) because the 20 amp pedestal breaker will not properly protect anything downstream rated at 15 amps.
This is a good mod. to do if you want more power for a heater or other appliance and it's fairly straightforward to do. I did it on a previous TT and the only difficult part (nerve wracking) was drilling a hole in the exterior.
Camping on the weekend, I noticed several MHs with a cord out a window plugged into the 20 recept. on the pedestal. No 50 amp pedestals in our CG so the only way to get more power is to utilize the 20 amp recept. Just looks funny to see a big dollar MH with a cord hanging out a window....