I switched out the T-stat on our previous hybrid and TT. Very easy to do. I like the Dometic T-stat in our current 5th wheel. Couple of links for you:
I got a value for our camper off NADA last week with no subscription required. I was shocked at how high the price they came up with is, I wouldn't pay that price. You can also try RVTrader to get a value. And do a search on craigslist to see what they're asking for comperable campers.