I have a Rheem electric heating system. The blower comes on but it's not hot air. All wiring in the unit is hot. Could it be the themostat?

Usually it is the

Heaters themselves

Defective heat sequencers or relays depending upon what your unit uses.

Tripped or open high limit safety controls.

These are the usual causes. The thermostat would be the last item I would consider.

Naturally there can also be other causes so if you have a electric meter and know how to use it get busy and track the problem down. There should be a wiring diagram on the back side of the blower access door.