theres a possibility that there could be something wrong with that 15A house circuit. But more than likely, theres a load somewhere in the trailer that exceeds 15Amp. Anything with a motor will draw a lot of current when it fires up.
You can get a curcuit tester at home depot, or wall mart for a few bucks. Its basicly a 3 prong plug with 3 lights in it, and a chart on the top that tells you what the lights mean.
The super-fancy ones have button on top to test GFCI circuits.
These are handy to have at strange campground to make sure their box is wired right and will help find out if theres anything wrong with the house circuit.
I almost dont want to recomend the following because, as a rule, Breakers usually trip for good reasons. In other words, they are a safety device and not a testing tool.
But you could turn off all the breakers in the breaker box in the trailer, then plug the trailer into the 15A circuit in question.
If the house breaker still trips then you are either looking at Ground/hot or neutral reverse like thecatsandi mentioned, or a big appliance thats not wired into the trailer breaker box
Both of these are could be dangerous. The former because your in a metal can, the latter because that means someone wired something in where they shouldnt have.
If the house breaker does not trip, then you could start turning on the trailer breakers untill it does.
That will at least give you an idea of which circuit the load is on.