Ground is ground. 2 amps is not a concern, but higher amperage might burn out the little tiny thermostat wire, not a concern here. Ground is ground, so if ground is issue with your pwm then any ground will be an issue. The steel camper frame is ground also, the entire frame of the camper is ground, all the ground wires are ground, they are all connected to the frame and to the negative post on the battery, ground is common to everything.
The only concern, as stated, is how much amperage will be flowing through your chosen ground wire.
I dont think there would be concern with your thermo wire, but try it, if it works then it works, job done.