[Espeefan]

JD, what you have there is an ESC (electronic speed controller) diagram, with some built in inputs for limit switches. Item 295 is the ESC that Ric recommended to you. You will use this one to run your screw drive, if you so choose. Your limit switch inputs are necessary because you'll want something to stop the motor running the screw drive, so it doesn't over travel at the top, and bottom, of it's stroke.

If you purchased a radio, it will come with a manaul, so most of your questions regarding the radio and receiver will be answered there, however to quickly answer your questions, radios have separate channels for each function. Steering and throttle are two separate channels. The screw drive will be another. On some radios, you can assign a toggle switch, knob, or stick to any channel you so desire, but this is a feature that usually comes with the more expensive, programmable radios. Most 4 channel radios do not have this feature. Your manual will tell you what channels are for what stick, knob, toggle switch, ect. You just need to figure out what you'd like to use. With that said, you'll be able to run the screw drive ESC with a toggle switch, or a stick. Which ever you choose. But be aware that a toggle switch will be either on, or off. A stick will give you proportional control. Slow to fast, and anything in between, depending how much you push the stick. It's up to you to decide if you want proportional speed control of the screw drive. It's personal preferance, but I think it would be fine with a toggle switch.

You asked about a BEC - a BEC is battery eliminator circuitry. Basically the radio receiver needs power to run. The receiver can be powered by it's own dedicated battery, or it can get it's power from an ESC, which has internal BEC. With the internal BEC, the receiver does not need an extra battery pack for itself. It draws power off the battery pack that runs the model. Hence the term BEC. Do you need a BEC? Most all ESCs come with some form of internal BEC, so the short answer is no, you don't need a BEC, if you have an ESC featuring that. There are exceptions to the rule. There comes a point where if you are running a lot of servos off the ESC's internal BEC, it may not be able to provide the current the servos are asking for. In this case you would want to get an external BEC. An external BEC is just a separate stand alone device which drops the main battery pack power down from a high voltage, to something less, so your servos don't get over volted (fried). For a semi truck that has one steering servo, and one shifting servo, you probably won't need an external BEC. The ESC should be able to handle that.

If you want to run two different ESCs on one receiver, you just plug each ESC into it's own separate receiver channel port. Easy. One ESC will run your truck's drive motor, and the other ESC will run the screw drive, for the dump box/trailer. The receiver will run each ESC separately, based on what you are doing with the controls. Again, this will be explained in your radio manual.

As for your choice in the screw drive motor, I can't offer much advice with that. I've never built a screw drive, so I can't help you pick out a good RPM motor. I am sure some others will chime in with advice. I will say you can run a 12 volt motor on less voltage, but you then need to account for a decrease in it's RPM.

Battery packs - you can run whatever you feel like. 7.2 to 12 volts is pretty common. I think it's more common to see 7.2 volt battery packs with the Tamiya trucks. Feel free to run either, but it's nice to stick with a readily available 6 cell, 7.2 volt pack, as they are cheap, and available everywhere. Not so with 10 cell, 12 volt packs. I know none of the hobby shops in my area stock them. You can get them online though, easily enough. My advice is to stick to the most common battery pack types. Either NiMh 6 cell packs (7.2 volts) or LiPo 2S packs (7.4 volts). Battery discussion could be a whole topic in itself, so for now we'll keep it short and simple.

Hope this helps.