If I have 14/2 wire I get a 15 amp outlet and a 15 amp breaker. If I have 12/2 wire I get a 20 amp outlet and a 20 amp breaker.

If I have 10/2 wire with a 30 amp breaker, whoa, I can't find a 30 amp GFCI (weather and tamper proof, its for a outside outlet). I have been using the circuit to keep my camper plugged in while building the cabin. I have been using a 20 amp gfci. To go code I thought I would need the 30 amp outlet to keep the electrical inspector happy.

When the camper goes, I plan on using the outlet for just normal use, no big draw of anykind.

I had one friend who told me to change the breaker and the outlet to a 20 amp, with the outlet weather and tamper proof, leave the wire as is, 30 amp. Is he a friend a guy can count on?

<cleaned up for readability - KISS>

A 30A receptacle DOES NOT need GFI protection, even if it is outside.

Your friend is right, change the receptacle and breaker after the trailer is gone. You CANNOT have a "standard" 15 or 20A receptacle on a 30A circuit, regardless of the wire size.

Tamper prrof had a different meaning for me and that's a new NEC standard, so kids don't get shocked.

In an electrical supply house, you should be able to find a clear plastic lockable cover that will allow a plug to be kept in use. Why not use a twist lock.

Why not go with a 30 A GFCI breaker?

You can keep the wire rated for the higher current. You can pigtail in the box to a lower AWG if the breakers or outlets won't accept the wire awg and change the breaker to match the lowest AWG wire.