We build Pole Barns for Ag. Buildings and commercial buildings. Most ag buildings the pole spacing is 10 foot on center likewise the truss is 10 foot on center sometimes you can get away with 12 feet. Most commercial buildings are 8 foot on center thats what can be used for residential structures is 8 feet. 6 foot center is a little overkill on an 18 foot span structure.
The roof system works like Terry said, 2x4's on edge on top the trusses 2 foot on center as you go up the truss. We use 60 penny barn spike thru the 2x4 and into the truss and a 10 penny toenail, thats what we need to use here in WI. The 2x4s should overlap each other approximately ten inches on either side of the truss and two nails on each overlap or they can be strung in between the trusses 2 feet on center.
We use 2x6's for the wall girts again 2 feet on center and those can be nailed with 3 or 4 16 penny nails.
The only bolts we use are to bolt the truss to the pole.
Squaring the corner posts will take some figuring. If yuo can sneek a 100 foot tape under your trailor would be the eaisiest and probably the most acurate.
Mueller Construction LLC.