-just a couple of notes from another person who has "been there, done that:"
Echoing what Vital49 has already said, I think he means the RTV to be used only at the four corners of the lower intake manifold gaskets. (-you'll see where the factory original dobs of RTV are currently.) -(I used FelPro gaskets through CarQuest, don't know if they are any better than the GM Dealer gaskets, but did look slightly different)
And, I would have stated, "classic case of LOWER gasket failure." The failure of the upper manifold, with the EGR stove pipe melting into the coolant passages will be discovered as a slow disappearance of coolant. AT ANY RATE, "tverhein" be sure to read the tech info about this. My bet is that your lower gaskets have definitely failed.
-be sure to post a reply to this topic, so as to let us all know what you found.
Also, if you replace the upper (plastic) manifold, I STRONGLY suggest you consider doing something to get the exhaust heat away from that plastic, otherwise you will be melting the new part too.
For what it'* worth, I experienced both failures, on my bonneville, a 1999 with right around 99k miles. - a car that had been well cared for too.
Also, echoing Vital49, coolant in the engine oil can mean certain death of the engine. -especially the crankshaft bearings. As a good measure, I replaced all the connecting rod bearings. They definitely showed coolant damage. (I would have replaced the main bearings too, but you can't do that with the engine in the car.) For a decent mechanic, dropping the oil pan, and replacing rod bearings on these engines is a pretty easy job. (for the average weekend do-it-yourself type, hmmmm,,, I'm not sure). Whatever you decide, do things carefully and "by the book."