You need to be testing what you plug your trailers into. There should be no reason to ground your trailer if the power pole is wire correctly. If the power pole is wired incorrectly, you are risking damaging all your electrical equipment as well as yourself. You can buy a simple inexpensive tester that will tell you if a ground in not connected correctly (if at all). Just plug it into a plug adapter and it will still work. this is what I use for the various campgrounds we stay in. You would be amazed at how many "weekend electricians" will rewire a campground pole (mostly in public campgrounds) because they have screwed up the wiring in their RV so badly, it won't operate on a correctly wired pole. They truly think they are "fixing" it.. $8 one shown
also has one a little cheaper at $4
Mine is older and yellow. Does the same thing. It will show the most common problems. I plug mine into these adapters... (not, this has kept us out of only three campsites in 10 years and possibly saved the lives of one family... their site was miswired and they were getting a "tingle"... then it rained)
15 mp to 30 amp. I can test a 30 amp plug which is what the bus takes. The tester generally stays in this adapter. But I also need the tester to test the 15/20 amp receptacles because we plug the food cart into the 20 amp plugs to charge the battery or run the 60 watt bulb we stick in the cart to prevent the water from freezing. My heat taped winter hose also needs to be plugged into a 15/20 amp plug. It doesn't need an adapter for that.
I can also plug my 30 amp RV into a 50 amp service. This is what we use for mobile home parks and some 50 amp only campground sites. The 20 to 30 amp adapter shown above will plug into the 30 amp plug of this and I can test the 50 amp outlet.
Like on an RV, when you plug your trailer into the correctly
wired power pole, you are using their grounding.
But what do I know. The Class C we were living in has a ground fault in the panel box and we were plugging into ground fault protected power poles (correctly wired). The RV ground fault would trip before the power pole. I've been told that won't work... but it does on our setup. And I'm not the only one with a working setup like that.