I baught an old trailer. It has an old coleman heat and air unit. The thermastat only has two wires.
So what controls the air? How does the air come on?

A two wire thermostat is for only one function, probably heat. You may or may not have an AC unit but you don't have a stat that can control both heat and AC

Hi
There can be two possibilities
1) u have heat only application
2) u have heat and cool both ,in this case u should have a heat cool changeover switch on the system it self.
In 2 wire system the fan is controlled by the system itself,the system advise the fan as when to turn on.
If it's a 24 Vac system then u just need to connect wires to R and W.