14 ply, 8 ply...... all over kill.
How heavy is your trailer??
Altho trailer tires are specific to trailer use and at some point a car tire is a bad choice, on an Expo trailer weighing under 2000# any 4x4 SUV tire built for a 4000# vehicle will work fine on a single axle trailer. Trailer tires are built specifically to handle the excessive twist as a tandem or tridem trailer turns a tight corner. On a single axle trailer there is zero need for a trailer tire.
Scale yer trailer, pick tires that will handle the load and air them up as the spec says. If you are running Load Range C tires the sidewall will give pretty realistic air pressure numbers. But if you pick Load Range D or E tires, the air pressure shown will be based on MAXIMUM loading which an Expo trailer is likely far under.
Read the load/pressure maximum and scale yer trailer. I run all my tires, trailer and Jeep at 26psi all year. Tire tread wear is always even, I never need to air down. They are Load Range D, 7.50R16s or in English they are 185/100R16.
I run the same tires on my Jeep and trailer. No issues, I get well over 75K kms on both the Jeep and the traiier tires.
PS.....
On weight.... ANYONE selling tires should know. Handling and ride and MPG are better with less unsprung weight. A heavy tire will ride rougher, the rough ride will be more damaging to everything on your trailer and altho it might be more theoretical and hard to measure.... a heavier wheel and tire will burn more gasoline per mile. Yer tire guy is an idiot.