30 amp is a 3-pronged lug that transfers 120VAC thru one set of cables into your trailer. This is the 'standard' for most RVs out there, unless you have a big MoHo or 5th wheel, where they are wired for 50 amps.
50 amps is a 4-pronged plug that carries 2-120VAC circuits for a totalof 240VAC. While nothing in most RVs run on 240VAC, it is there if it is required. 50 amps allows you to run 2 air conditioners (3 if you have them, and he larger MoHos DO) and almost everything else in the coach.
Now to your question. If your trailer is wired for 30 amps, you can plug into a 50 amp receptical at the CG with the use of an appropriate adapter. You willstill be drawing 30 amps, even though you will be connected to 50 because you will only be accessing one of the 2- 120VA circuits available.
Hope this helps.