What PSI for tires?

Over the last 3 decades we have normally filled our ST tires on our towables (TTs, 5er, toy hauler) to max psi as stated on sidewall, although we usually had them about 5psi below max to reduce bounce when towing. With our LT tires and our class A tires we can choose to use actual weights, that being said we still air them up regardless of weight about 5-10psi below max, that way our tires have plenty of load carrying capacity while reducing the harsh ride that max filled tires can present. Never had a blowout or catastrophic tire failure while RVing, and yes we have been over the 65mph threshold here and there pretty much every trip. If your 5er has ST tires best to fill to or near max psi for best performance, LT tires can be filled to only handle actual weight of rig, again we fill close to max psi for added weight carrying capacity.

There is a recent thread on TTs about nitrogen filled tires.

We don't know what tires you have on your FW, or the GVWR of your FW. In most cases, if ST tires are original, or replacements same as original, you would air them to max sidewall pressure. Reason being, the manufacturers usually install the least, but just capable tire for the trailer. In this case, that tire is only capable, using max pressure.

Jerry

Generally speaking, if you're on the tires that came with the rig- fill them to whatever the tire placard on the side says.

If you've upgraded tires, go by tire inflation load tables (maybe with a 10-15% increase to account for differences in side to side weights).

As for putting air into nitrogen tires, air is already 78% nitrogen (according to Google). Just top them up and you're fine.