Funny, I saw Turp's road trip review. I took my new truck up to the Sierra this weekend on it's first mountain venture and was planning to do the same.
I have a 2011 AC PreRunner with the 4 and an auto tranny with a Leer 180 cap on the back. I have a heavy memory foam mattress in the back, plus the usual camping gear and the dog. Overall, I am very pleased with the way the truck performed.
I went up to Bishop creek, above Bishop for those of you who know the terrain. The climb out of Bishop into the high country is fairly steep at times. The truck kicked down, but climbed admirably and I had no problems keeping it at 50+mph in the steepest sections without really winding out the engine. It actually did far better than I expected with all the climbing.
On the drive up to Bishop, going up I5, the 14 and 395, the cruise control is a little quirky when going up moderately steep grades. With the cruise at 72mph, the truck would slow a little, kick down and then really kick down (about 5000 rpm), go up to 76mph, then drop back. I learned to turn off cruise on those grades, then turn it back on at the top. Otherwise, it handled the drive quite well.
My mileage on the way up was 23.7mpg (no wind) and coming home into a headwind the whole way, about 22.3. In the 65mph sections, I was doing 72mph on the speedo, which is really about 70. I was not driving particularly easy.
All in all, I am very pleased with the truck's performance.
As an aside, I am very glad I went with the Leer 180, the mid-high version of their cap. I have plenty of head room sleeping in the back (I am 6'1") and there is no sense of claustrophobia back there. It feels quite spacious (I laid down in a friend's cab-high cap and it felt a little like a tomb to me).