It is most likely the shore power connection. All three wires from the shore power has to be used. Not enough info submitted to really guess at much...
Are you connected to your garage at home when you are getting this small electrical shock. The three conductor coming from the garage must be in good condition if you are using a RV30A-15A Adapter plugged into a good 120VAC garage receptacle.
Your trailer frame is only grounded through the GREEN WIRE coming from the shore power connection.
This is a typical 120VAC wiring connection for using 50AMP and 30AMP connections feeding a RV Trailer
sample photo from GOOGLE images
Need more info on how you are setup...