Shrink tube will melt. Besides, unless its the stuff with an even lower temp softer inner core that seals it will leak anyway as most the cable used for most temp probes has a braided outer jacket.
I find that Permatex Ultracopper high temp silicon works well for lots of things around the smoker including sealing up temp probes. I also thread on a small coil spring around the area for strain relief then goop the silicone around the area the cable exits the body of the temp probe and the spring holding it in place. Let it sit up overnight just to be sure and you should be good to go.
Ultracopper is rated for use up to 700 degrees F. If your smoker gets that hot, the temp probe will be damaged beyond repair in any event so moisture intrusion would be the least of your problems.
The quick check for themometers of all kinds is a water bath of boiling water 100C (212F) at sea level and 0C (32F) using an ice bath, water and ice or slush.
Of course the more important temp for a smoker thermometer is the 100C point. The boiling point lowers for higher altitudes. Here's a chart to use as a guide for your locale. It would be close enough for a smoker to simply interpolate between the altitudes listed here.