William, I have never used RTV on o-rings....just petroleum jelly....

Also, before putting that tensioner back in, make sure you clean the passage ways where those elbows go in with some crocus cloth....

Personally I have never used it either, I don't like to take the chance of any of the stuff getting into the coolant system, but others have claimed that it helped keep theirs from leaking, and have said is poses no harm, so since they did not say they were going to get the elbows I suggested I figured I'd mention it as extra preventative against the cheap dorman o-rings from leaking.

Good point on getting the bores clean, I planned to soak mine in degreaser, so I carefully used 0000 steel wool, but that is not something I suggest others use as if you are not careful you can scratch the aluminum up, good tip on the crocus cloth!

1.
is the installation of a belt tensioner the same for a buick le sabre 2000?

2.
you don't have to take off the plenum(etc) to put in that elbow that connects from the belt tensioner to the intake manifold ?

3.
are the o-ring included with 47065hp dorman good ?

Yes the procedure to install it is the same.

No you do not have to remove the plenum to do so.

From what I have heard they addressed the o-ring issues, even with the current nylon elbows, but I'd always recommend getting the aluminum elbows over the plastic ones.
Let me see if I can find the info about the o-rings, and the AC Delco O-rings that I am sure are the ones that are on the OE coolant elbows.
If you get the aluminum ones you should not have any issues with the o-rings as I mentioned, I have not heard of anybody that has used them have leaking issues.
But in case you are unsure about them I am almost positive these are the o-rings that are of a better quality than the ones used on the old plastic Dorman elbows, part number 1534066
ACDELCO Part # 1534066

I was unable to find the info I had read before about Dorman issuing a fix for the plastic elbows with o-ring problems, it does not really matter though, because the aluminum elbows are the way to go.

i used the aluminum ones, they have been on mine for over a year and dont leak. i put a little permatex ultra black on it around the oring area for a little extra protection. and you have to make sure all the chunks of the plastic ones are out of the hole before you put the new one on