Bigrigger, I'm looking forward to seeing some progress on the trailer. A half round end dump would be really cool to see. I never really thought about using PVC pipe for that. That's a great idea.
With my limited styrene, and body filler experience, all I can say is I hate trying to use filler. I've never gotten satsifcatory results, ever. The problems I've always had were because the filler cracked. Cracking is almost unavoidable when you have a plastic cab that can easily flex, and the putty cannot. It's best to build up low areas with styrene, to eliminate the needs for thick coats of filler. I've used a technique much like this one. Using this method, you can build up material, file, and then sand it smooth to the surrounding area. Like filler, you can also go back and add more, if you need to.
http://www.youtube.com/watch?v=2m9rVrVNa6A
Also, roughing up the places you want to coat with some filler is a good idea. It cleans the surface and provides some teeth for the filler to grab. Just hit those spots with some 100-150 grit sand paper first.