He's correct -- Trailer tires
typically have higher max psi than auto tires!
Also recommended to use valve stems with higher psi ratings than car stems (usually metal body inside).
Impress him with your new knowledge of other differences between car rims and trailer rims:
1. Car rims may have the wrong off-set (Distance between vertical tire center and mounting center as viewed from rear) -- Trailer off-set is zero or very small.
2. Car rims usually fit tightly to the hub and are centered there (aka Hub-centric) with the lug nuts/bolts only holding the rim to the hub. OTOH, trailer rims fit very loosely to the hub and the nut/bolts do the centering (Lug-centric). You can see the difference by looking at a car hub (tight fit) and a trailer hub (gap).
This last is important if you want to get the trailer wheels balanced and the shop doesn't have the LC adapter -- If there is a difference between the stamped hole center and the lug center then the wheel with be off-balance...
BTW, I personally dislike hub caps on trailers because they don't let me see, feel or shoot the IR temperatures of the hubs when traveling.
They remind me of dust ruffles which allow monsters to hide under the bed where I can't see them...