Spread axles are generally spaced at 10'-2". Like Flatdeck said, rear axle is usually where you find it on a 48' trailer. On a 48' Trailer the rear axle is normally 28-30" from the rear of the trailer, so you would tack the extra onto that. That is why 53' trailers usually have such a long overhang (see pic) Kingpin is normally 30" from the front and legs somewhere around the 130" mark. Trailers are generally 102" wide, you won't get the deck height quite right because of the truck's 5th wheel ht. Remember, divide dimesions by 13.5 and not 14 to get the true scale of the tamiya truck.
Hope this helps,
Chris