Since the trailer handled OK behind your PU dually and the MH does it with and without the trailer, it's the MH or as lanerd implied... possibly the driver of the MH. Monaco's RR8R chassis is noted for their good handling, even with more than 10K behind it plus you had it aligned without a change. Coming from a PU and assuming your the driver of the race car, it's natural and easy to oversteer something this long... even after a year. With practice and no wind it's amazing how little steering input it takes to keep it straight, but you have to give a small input three times as long to take affect. Looking further out will help, but once you start chasing it, especially with a long trailer on the back the harder it gets to stop oversteering... NO offence as even after 12 years on a really windy day I'll sometimes have to remind myself to stop fighting it and give things a little more time.