dude, anyone charging in with an uber kills a sentry. there's no way to make a sentry immune to an uber, it's the counter for it. But the stairs on CP1 of dustbowl will pretty much stop anyone else until they get that uber going. you just have to accept that there's no way to make a sentry impervious unless the entire enemy team is scout/pyro. and what I meant about the sewers for 2fort is that you put it in the back corner, actually. that way a demo/soldier can't get line of sight without the sentry being able to fire on them too. it's not flawless, but it's only going to get taken out if they get some quick shots and a few of them are criticals.
there is no real sentry spot that makes it impossible to take out, unless someone changes the gravity, and even in that case there's probably sniper/rocket access to take out a sentry. Just put it in good spots where they have to change the way they move/attack on the map and then the other team is funneled into another route until they get an uber or a sniper in place to take it out.
the main thing with setting up sentries is you want them in a spot where they're only accessible close range, and it's really tough to do that on every map and still have one put in a good place to get kills.
Although, this does remind me... the final stage of dustbowl and its last CP. down on the side of the tunnel entrance, there's that little nook. building a sentry there is good. If you place it just far enough back anyone who comes out of the tunnel will die, so it'll cut off that sneaking lane usually taken by pyros/other adventurous people so, once again, they'll have to go get an uber built up.
Just don't expect a sentry to survive an uber, because even constantly healing it while ducking behind it won't outlast the typical ubered heavy/soldier/pyro. The worst though is when the enemy team coordinates and has a spy rush in and sap everything, then an offense guy rushes in and unloads on your stuff while you're trying to knock off the sapper and you more than likely die unless they're really bad.