Lots and lots of tire pressure threads out there, but in summary, check your tire pressure BEFORE you start out your day of driving.....whether it's in the morning or if you are leaving in the afternoon or evening....or even at night. The important thing is to check your tire pressure before the tires have had a chance to heat up from driving. Tire pressure will change as ambient air temperature changes, so the term "cold pressure" refers to ambient air temperature....not any particular temperature. So check them at their cold (not having driven with them) and set them at the recommended tire pressure that is listed on the nomenclature plate that is on your camping trailer. And most likely, they are NOT filled with nitrogen from the factory. Keep in mind, that if you set your tire pressure in the morning, and during the course of the day the temperatures (ambient air temperature increases), your tire pressure will be higher than where it was set that morning. This is perfectly OK as the tire manufacturers have engineered and built the tires to take that into consideration. Hope this helps.