How to patch a hole in the coolant reservoir tank?

Neither of those products melt plastic. I've used Chrysler's RTV gasket sealer for a huge hole in my mother's gas tank, and a variety of other projects including gluing in plastic switch panels in car radios. The people at any Chrysler parts department have two RTV silicone products. The black stuff remains a little more rubbery and is easier to scrub off, but it absolutely will not bond or seal through oil residue, such as when reinstalling a transmission pan. Their gray stuff cures a little harder, is harder to clean off later, and will bond through a light oil film. You'll want to lightly sand the plastic first so the adhesive will bond better.

RTV gasket sealer is not a good choice for anything with pressure in it. Most GM models, for example, have their radiator caps on the coolant recovery reservoir, so it is under the same pressure the cooling system is. That can be as high as 18 psi. When that is the case, epoxy is the better choice, but I'd still consider it a temporary repair.