Presumably you let the camera and lens cool down for a few hours first?, if not, cooling down when bring a warm camera and / or lens outdoors at night will condense water vapor.
It matters what the environment is (I'm a meteorologist, so I hope I can contribute something since I've gained so much photography knowledge from this site). If it feels really "humid" or muggy, then cooling down the equipment is going to present significant problems. If the dewpoint is above the temperature of the lens and camera equipment, then dew is going to form on the lens. This can happen (and has happened to me), for example, if you put your camera equipment in your air conditioned car to drive to your photo location before taking it out into a high-dewpoint environment. In this case, your lens will be analogous to a cold glass of water on a humid summer day.
I suppose it may be possible to have the opposite situation occur. If you change lenses in a humid environment (by "humid" I mean one with a high dewpoint temperature), you may have issues if you move outside into a cold environment. The camera equipment may cool enough to lead to condensation on the rear element as the "humid" air trapped beyond the lens cools. This doesn't seem very likely, though, unless the mount and camera is air tight to prevent the "humid' air from leaving the camera / let the drier/cooler air from entering the camera.
I don't have much advice for the OP since I haven't run into this problem. Actually, if the cause is from a cold lens/camera on a high-dewpoint night, then yes, I have experience this. I tried to shoot some 4th of July fireworks pictures a couple of years ago when the dewpoint temperature was near 80 degrees. Since my camera equipment is stored in my house (which was around 75 degrees) and I used AC in my car to drive to the photo location, the temperature of the lens was less than the dewpoint temperature of the outside air. As a result, condensation deposited on the lens, and I ended up with a worthless, hazy photo session.
If this is the cause, then I suggest trying to heat up the equipment in your vehicle first. For example, set the lens on the dash and blast heat from the windshield defroster. This will limit condensation, at least until the equipment cools down to the ambient air temperature (it's not uncommon for relative humidity to reach 100% at night under clear skies, when radiational cooling allows the temperature to cool to the dewpoint temperature). .