Ok wow that might be way to Highpoly for source engine dude, unless you wanted to normal map it maybe. I mean if the trailer part is 6000 tris i wouldnt even want to know what the WHOLE vehicle will come out to, that trailer alone has as many polygons as HL2 player model let alone a vehicle prop, which im guessing would average around 2000-3000 polys at most, give or take.
So my advice, are you modeling using subD(subdivide)? If so id reccomend modeling it by hand(box modeling, ect). Yes its alot harder, but thus is the nature of game art.
Tips-
-Those cuts stretching over the top of this trailer seem a huge waste of polys.
-The tires also seem WAY to highpoly. Especially the "rim".
-The handle on back of trailer doors seems way condensed. You could acheive the same look with a 1/4th of those polys
-The floater on top of the thing again is WAY over condensed, again could be done with a fraction of those polys.
Not to rip on your model, its a GREAT concept, but you could easly cut your poly count in half while acheiving the same sillohuet shape, yeah it wont look as "smooth" or "highpoly" but no one will notice once you apply smoothing groups and a texture, that thing in game would bring your FPS to a halt ;p
Overall, try to make it as low poly as you can. Look at the wireframe for the 2fort truck as reference, barely any polys.
-Buddikaman-