I began with the passenger rear wheel, followed by the drivers rear, passenger front, and driver front. Are there any other bleeding locations? I've read that proportioning valves can still have air trapped in them. I'm getting no air out of my bleeder valves on the calipers, but my pedal still feels a little mushy. Any insight?

To properly bleed the system you should bleed the Right Rear then the Left Front. (That is one half of the system) Then the other half, The Left Rear and Right Front.

The two systems, if you will, are cross ways.

Did you start bleeding by doing a gravity bleed first. That is, open the bleeders and remove the top of the Reservoir to allow gravity to bleed the fluid and air out of the system.

Then, when doing the 2 person bleeding by applying the brake pedal you should follow the above mentioned sequence.

Some people have had trouble getting all the air out due to the ABS module trapping air and it can take some time. I have had good luck with gravity bleeding taking 90% or more of the air out and the remainder under foot pressure.

If still having problems you may need to buy a vacuum bleeder from the auto parts store or Sears for about $20.00 to $30.00 and vacuum bleed each caliper by hand pump.

I have yet to need my pressure or vacuum bleeder on the Riv and I have bleed the brakes after the system has been cut open and drained at least 3 times since I've owned it.
I just bleed it as I've mentioned and it's done.