Carla, the more I think on this, the more I wouldn't recommend cutting the locking ring off if you get this adapter (which you could cut the handle off no problem)................or in using any adapter that doesn't have a locking ring already on it.
The reason I say this, is the NEMA L5-30R connection (the part that twist locks to your RV) really needs that locking ring on it for two purposes.
1. The locking ring will help hold the connection in place. If you don't do the actual little final twist all the way, or it comes loose, you can get what is called electrical arcing across the connections in the cord and trailers inlet. Over time this can ruin either or both. There are several forum threads that show where other members have had this unfortunate problem.
2. The locking ring also helps hold the connection tight to prevent rainwater from entering.......which you also don't want water on an electrical connection.
The RV place you picked this up from may have had a connector in place to quickly run electricity for your PDI, but from the way you describe the connector they were using, it wouldn't be recommended for long term use....like you are about to do....which sounds like you are going to keep it plugged in while near the neighbors fence.
Just a little heads up on another thing, but it may or may not apply to you (if you don't have an air-conditioner in your RV). If you do use the 30 amp to 15 amp adapter (that let's you plug into a regular household outlet/extension cord), then you are limited to using less amps (15 to 20 usually). You may not want to use an Air conditioner (if equipped) when plugged in this way.......as it could ruin the AC or overheat the connections/cords. I know in the dead of winter, most people aren't thinking of Air Conditioning, but warm weather will be here eventually.