And P.S. the manual likely won't have anything to say on the subject. RV manufacturers usually don't produce manuals for each model - usually just their entire product line, where things are described in vague, general terms. There are therefore too many tire possibilities to cover so they will just refer you to the aforementioned label...
which looks like one or both of these on the forward end of the driver's side.
Of course, the label is only correct if the same spec tires are now in use (you say you bought the trailer used so you really should check that against the label). In my case, I've upgraded from load range C to load range D so I now use a cold pressure of 62psi (max 65 on sidewall).
Seeing that its a 2011, many will say you should start thinking about replacing the tires so you may want to consider going up a load range if the loaded trailer's net weight is closer than 85% to the sum of the tire load maximums.
2015 Rockwood Signature UltraLite 8282WS Platinum, GY Marathon LRD, TST 507RV TPMS
2005 GMC 2500HD CCSB D/A, Curt E16, Prodigy P2, Garmin RV760LMT w/BC-20 b/u cam