If you have a 3rd wire to the slide motor, it is usually an orange wire. Orange is an accessory color if they use the standard colors. The 3rd wire if it goes to an accessory should have a breaker or fuse inline somewhere. It could also be a wire to power the break-a-way switch, which are not generally fused (if it's a break-a-way, it is probably black also but will be smaller diameter).
Red/Black - Red is positive, black is negative; Black/White - Black is positive, white is negative. Oddly enough my trailer has Red positive and White negative from the battery. So there is a third possibility.
Yes, hook up the positive and then follow with the negative. Logic behind that is if the positive is being hooked up and there is no negative connection yet, if you wrench hits a grounded part of the trailer frame there is no sparking as there is no complete path back to the battery (frame is ground normally and you have not hooked up that wire yet). But if you hook up Negative first, and then your wrench hits the frame while hooking up the positive, you have a path from the positive direct to ground from the wrench which is touching the positive terminal via the frame to ground/negative and poof - arcing and sparking takes place. It's probably more likely in a automotive environment, but it "could" happen in a trailer and is best practice to do it that way.
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2015 Freedom Express 246RKS
2002 GMC Sierra 1500