Gary, I run mine at MAX pressure stated on the sidewall of the tire (50 psi) ... most trailers are fitted w/ tires to carry the trailer's MAX load at the tire's MAX pressure ... take a look at the sidewall and see what it says ... let us know...you know how curious we are ;) ...
I've got 13''s, too on a tandem set up w/ Goodyear Marathon Radials ... MAX pressure will give you best handling AND mileage (fuel & tire wear) ...my trips from SC to FL (11 hrs) and from here to coast (6 hrs) have been w/out incident, so I feel pretty confident w/this from experience :) ...
...course I do put the top down ;) ...