I have used Tunnel quite sucessfully for a few surveys but it was a challenge to use it sometimes. I got a lot of help from Footleg and Julian Todd and I have just been looking for some old emails from them about elevations but I can't find them.
I just looked at some survey data to try and remind myself what I did and I seem to have two SVX files, one is the usual plan view and the other is the same but with the following at the top of the file:
I've just tried it in Tunnel and if you have the above line it does indeed import it as an elevation. The ";" at the start is important or it won't work.
Hope that helps or gives you a pointer.
I know Therion is better but I'm too lazy to learn something else for the amount that I use it.