RV Outlet NEC Code Question

Do it correctly the first time and in the long run it will be a lot cheaper. You can buy a good 50/30/20A breaker equipped RV box for about $120. Add the appropriate size direct bury cable with plastic conduit as it comes out of the ground (this should be legal in most areas but check your local code) and be done with it. You will always find that saving money up front will cost you more down the road. With the job done like you were setting up a new RV park it will last you for the rest of your life and will be there if you upgrade to a 50A RV. I myself always use a 50 to 30 adapter cord to my 30A EMS if the park has 50A outlets in the pedestal. The 50A outlets are always in better shape and much less likely to be burned up compared to the 30A outlet. If you can not afford it right now wait a little while. You will be much happier in the long run.