It shouldn't be "cold" there even with the cool front we've had the past several days. It can get cold in winter with a 30 mph wind, but we're heading into summertime now. During the day there's usually a gusty 20-30mph wind coming off the gulf, so that helps keep the heat down somewhat, but yes as summer sets in it will get hot just like everywhere else in South/Central Texas. You'll definitely want shade, drinking water and sunscreen.
I would worry about mosquitoes. When the wind dies down after dark, the mosquitoes come out in full force. I welcome the refuge of a tent's walls when that happens. If you can handle the bugs (a blanket over your face and the rest of you in a lightweight sleeping bag?) perhaps it'll be fine. I assume the air mattress would be in the bed of your truck? Otherwise, you're liable to wake up face-to-face with a coyote. Every night we've ever camped at Padre Island NS, we've awoken to find coyote (and ghost crab) prints all around our tent. They don't do any harm, although the park service advises people with small pets to keep them corralled/inside vehicles at night, just in case. Also, you want to stay near your vehicle particularly if parked down island, since traffic on the island needs to be able to see you and you won't be very visible if you're on an air mattress on the sand away from your vehicle.
Hope this helps! It's been awhile since I've camped at Padre Island NS, but we've flown over it relatively recently. There are a few more (barely) offshore oil drilling rigs visible from the beach now... :rolleyes:
Not sure why you're having Google Maps problems -- here you go. The actual park entrance is a ways further down that road from where Google Maps pinpoints it, but it's enough to get you there. Once you're on that road, just stay straight until you see a huge parking lot on your left. That's the park headquarters, with showers, interpretive displays, NPS rangers to get info from, etc. Leaving there and heading further down the road, it'll eventually end at beach sand, and voila, you'll be driving on Padre Island National Seashore. Need a 4x4 if you plan to go far down island -- there's only a mile or two of non-4x4 accessible beach, and wrecker service is expensive if you get stuck. We never have, but we've seen plenty of people who thought their sport's cars and sedans could make it.
http://maps.google.com/maps?q=20301+...2658&t=k&hl=en
From there you can do a From->To calculation based on your address and the park's (which is supplied in that URL)