Thanks all for the quick responses. He already has a 30 amp breaker in his shed (for his air compressor), does it mean that all I need is the adapter and then plug it into the same receptacle as his compressor? Im sure I will need a heavy duty extension cord as his shed is about 25' from the trailer (not sure how long the trailer cords are usually).
Generally a air compressor will be 240 volt.
HINT: if it is a BREAKER (singular) meter the outlet to be sure
If it is 2 breakers joined to gether it is 240 volt don't use it.
(NOTE: single.. Well meter it first to be sure.
RV's have either a 3-pin plug (120 volt) or a 4 pin (3 flat blades and a round pin) I'm not going to explain that hook up in this post.
If I were you I'd have the electrician install a 50 amp RV circuit, tell him to wire it for 50 amps, but only use 30 amp breakers, This complies with all codes (you can always use a smaller breaker) and makes upgrade in the future a matter of deciding if you want to change the breakers or just forget it.
PLUS the odds of an electrician installing a 50 amp RV outlet wrong very very low, The odds of him doing a 30 amp wrong MUCH MUCH Higher, many threads in the forum about that.
IF you do have him install a TT-30.. Again, the advice above about METER IT FIRST.. Make him show you the voltage on the outlet (Flat slot to flat slot) and if it's not in the 110-125 range, (IE: if it's over 200) Then it's wrong.
Nothin adds excitment like something that is none of your business
Kenwood TS-2000 housed in a 2005 Damon Intruder 377