Actually, you'd be mistaken there. The 2500 trucks are extremely durable. Mine is at 195k miles, is all original, and I've spent like $300 in repairs over the last six years. Total.
I wouldn't hesitate to hitch up my 8600-lb trailer and drag it across the country with 100% confidence. Oh, wait, I did that a few months ago.
Yes, $11k is a bit high, but $10k is entirely reasonable, and they'd get it.
2007 was the only year with the 4L80. 2008 and up got the 6L90. Ask me how I know.
And that listing? A 2010 with 120k for $12k? That's about 10k too low. First thing I'd ask is, "What's wrong with it?" I'd expect to see a salvage title at that price.
The 2500 trucks are unicorns. Less than 10,000 per year were made, and 2013 was the last year. They are highly coveted in RV circles and command quite a premium. If I had to replace mine tomorrow, I'd budget $20k-$25k for a 2011-2013, well-optioned, with 100k miles.