Reread the tire.. It does not say just 120 PSI max it says something like this.
Maximum load 5,000 pounds at 120 PSI max pressure
(note weight picked out of air and is used as example only)
The only way to insure proper pressure is to scale the rig, each wheel (not tire if you have duals but wheel) and use the tire maker's web page.
On trailers if you like you can over inflate a bit, but still I don't recommend it. Here is why.
With the proper pressure the tread lies flat on the road, WIth over inflation the center of the tread is on the road but the edges are not, this leads to loss of control and a rougher ride. Under inflate and teh center of the tread is not working as hard and what's worse the sidewalls are flexing more, this leads to a loud BANG and a wop wop wop and brand new plumbing and wiring and wheel well panels where the tire tore the (term deleted) out of your trailer.