Taken from
UL's 651 Schedule 40 and 80 Rigid PVC Conduit and Fittings
1.10 Rigid PVC conduit and fittings covered in these requirements are intended to be joined to each other and to rigid PVC boxes, conduit bodies, and fittings in the field
by means of a cement that is or contains a solvent for polyvinyl chloride.
1.17 Externally-threaded adapters (also referred to as terminal adapters) covered by these requirements are fittings intended for joining a length of rigid PVC conduit or elbow or other bend to:
a) The knockout area of a metal box with a metal locknut,
b) A threaded metal hub or fitting on a metal box,
c) A threaded hub on a phenolic box, or
d) A knockout in a phenolic box.
So, per UL, we can connect PVC conduit or fittings to a PVC box with cement, or to a metal hub in this wet location. Sealing locknuts and washers are for metal conduit, unless otherwise listed on the carton.