Lots of discussions about this in the HMMWV forum (exact same wheel/tire assembly used for both).
only one I found mentioning stock offset was this
The stock HMMWV wheels are +63.5 offset. 9" rim width, 7 inch backspacing = 2.5" + offset = 63.5mm+ offset
(as far as I know, the math is 1/2 the width of the rim (4.5"), then use the difference between the backspacing and the rim width (9" rim, 7" offset) and subtract the difference from the 1/2 rim width (4.5" minus 2" equals 2.5" offset)
if it's closer to the outside face of the wheel, it's positive, and if it's closer to the back side (so the wheel sticks out further) it's negative.
negative gives you a wider stance, but puts more stress on the bearings.
For a trailer though, as long as it's not sticking way out past the fender I don't think you're really going to have any issues.
The axle is rated for something like 7k or 8k lbs, while the trailer itself is rated for 4200, so not really that much stress on the bearings (unless you plan to load it up really heavy past the military ratings)
the -18 offset seems pretty popular with the HMMWV crowd for 9" or 9.5" wide rims.