Help with my new (to me) Caribou

Most likely problem with the heater is a bad thermocouple. They tend to corrode, and then they won't let the gas valve open. Replacing one is very easy and inexpensive. If you want to check to see if yours is good, take it out and connect a voltmeter to the leads. Heat the end and see if you get any voltage. Depending on the type of thermocouple, you should get a few millivolts. The color of the leads will tell you the type of thermocouple you have ( probably a type K). Omega and other manufacturers have charts showing this info as well as telling you how much voltage you should get as a function of temperature. Now that it's out, I'd replace it even if it looked good.