So sorry to hear about you dog, I lost one of mine last year.
As to the question about making a mold and the RTV you have. Be sure to mix the body/main part of the silicon, be sure there is no solids on the bottom of the container. Next and most likely the problem, is the activator. They tend to be the first thing to go/not work. If you can, get a new sample of the activator and mix a small batch to see if this resolves the issue.
There are also silicon “putties” that you could get and use. May be easier using a putty than a liquid for this sort of mold. If you do use a liquid material, you could also use a thixate that may be available for your brand of silicon. There are other things you can do, more than you probably want to hear about.
One other thing that could be done is a warm clay or even wax could be used to make a mold of your pooches nose and that impression can be used to carefully make a cartable wax, but most likely you will only be able to get one wax from it. You could also use some silicon from the hardware store (the stuff in a tube) and use it to make a mold, just be sure to really work the material into itself and onto the surface, as it is thicker than regular mold making silicons. No matter what you use, I would recommend using some sort of release before applying any mold material, it could be very tramatic if the mold material started to stick to any of the flesh. Soap, light coating of vaseline, cooking oil, grease, something to make sure the mold will release easily.
Again I am sorry for your loss. Sad but think and keep in mind the terrific times you and your pooch had with one another.