I have a similar trouble with the camper I'm building, All I can come up with is the little mongrels are climbing the wheels and jockey wheel..
So I'm looking at a dish with a center platform as a rain cap [ wider than base ] that that the wheels will sit on top off .. the bottom ground plate dish will have oil in it about 5mm deep ..
Think of two stainless / plastic / tin plates [ sitting on a larger concrete paver pad, keeps the grass away from around the wheels ] one right way up and another larger one upside down spaced 20mm or so apart, with the three wheels sitting on top of them ..
Ants try to climb over the bottom plates and get trapped in the oil / rain shouldnt be a problem as the bigger top cap should sheild the bottm plate .. Onl down side is jacking the trailer slightly to pull the plates out when you want to move it .. cheers: