Reinforcing what Itch already said, you want to cut the fat from the model now before you shape it out.. It's alot easier to shape a model if its got less verts to jumble together, esp if most of them will end up removed in the end. Because this game doesnt use a draw distance type of deal it means pieces while attached to the model base do not have to be made of one solid object. The game won't "clip" away pieces which "float" over the body, so things like equipment, lights in particular, antennas, mirrors etc don't have to be modeled into the core body of the vehicle. They can instead float into the body saving a ton of polygons and effort in shaping those areas where they would attach.
Door lines do not have to exist in this game, and really should not exist because it's a ton of waste polygons. Instead expect to texture that seam line on since it can still look good but won't waste weight in doing it that way. The curvature on the cab and the bumper both have way too many polygons.. If it's a relatively "flat" shape don't put in too many polys, save the polygons for where they have to be used. Takes a good while of practice and trial and error to figure it out but given time you'll get it. Some things have to be more detailed than others, but often times you can get away with skimping and it still will look fine in the game.
I attached an example of what itch and I have spoken about to help you understand it, hopefull it helps give you an idea of what we were talking about.
http://www.emergency-planet.com/uploads/monthly_2016_06/ex.png.fab873a5451435339cb7f6dbdf4deda0.png