The only ground your RV FRAME gets is through the SHORE POWER connection...
Assuming you are plugging into a regular 20A or 15A 120VAC receptacle in your garage etc... you have to plug into a fully functional 120VAC receptacle that is using all three wires for the 20AMP-15AMP connection.
Your adapter cable RV30A-15A may be missing the ground wire... I always carry one of these RV30A-15A LONG Dogbone type adapter to use. Those small round adapters get too hot for me to use and may burn the contacts... I also never use any extension cord less then 12GAUGE (12-3) and actually use a 10GAUGE (10-3) cord. The 14-16 Guage cords are just too small of wires to use...
The extension cord you are using may not be using the ground wire - some idiots cut those OFF for some reason...
I always use one of these 120VAC circuit testers from LOWES and it tells me right away if the 20A or 15A receptacle I am plugging into is OK and if I plug this into the end of the extension cord I can tell if it has all of those three cables wired correctly...
What may be just a small tingle to you could possibly do more harm to someone else or even your beloved dog or cat...... Not to be taken lightly...
I would unplug my trailer from shore power until I got this problem resolved.
Roy Ken