There isn't a standard method of determining site size, so it's going to be hit and miss.
I believe Motorhome (not RV) lengths generally assume a TOAD which will be a few feet shorter than a pickup.
Also, I believer our long bed 4 door is around 18-20' but a short bed regular cab might easily be 4' shorter.
Also keep in mind, once you unhook, you don't have to keep the truck in line with the trailer. On smaller sites, we often park 90 degrees to the trailer, so we only need enough space for the width of the truck. Or if the front of the trailer is high enough you can park under the overhang.
Sorry there isn't an answer that will work every time.
