Well I had an interesting time yesterday sorting through my brake system and learning all kinds of things about electric trailer brakes that I never thought I wanted to know.
First, I did pull off and inspect my old magnets, way too worn to consider reusing. I did test them both electrically though and they are actually ok. In the process I found that my theory that buzzing indicates a bad magnet was dead wrong. Its normal when they are connected to the brake controller, which must be putting out a pulse width modulated voltage to do its control thing. Buzzing disappears when connected directly to a 12V dc source.
Next I found that the magnetic fields generated by the magnets themselves were sometimes confusing the readings on my dc clamp on meter. DC clamp ons use hall effect sensors which are pretty sensitive. So, I cut the wires and went back to using an old fashioned current shunt to do those measurements.
End result is that both the new magnets are likely within specs and don't exhibit ground faults. The passenger side one has about 5-7% higher resistance than the drivers side (hard to measure exactly because they heat up a lot when on and copper resistance increases with temp so the current goes down). I also have about another 2-3 percent loss in the longer wiring going over to the pass side. So overall the current through the pass side magnet is about 8-10% lower than the driver's side.
Nest I reconnected the pass side brake, left the driver's side disconnected, and attempted to burnish/burn in the pass side brake. My thought was that I had never really been able to get the pass side heated up and burnished before because aggressive trailer braking resulted in the drivers side getting plenty hot while the pass side was barely warm.
That seems to have improved braking action on the pass side so I reconnected both sides and did a few test stops to measured brake temps. I'm still quite a lot hotter (say 200 vs 140 degrees F) on the driver's side but its better than before.
So my conclusion now is that the 8-10%-ish side to side magnetic force difference must have been enough to keep me from being able to properly burnish the passenger side brakes. It surprises me that 10% would make that much difference so there certainly could be something else still going on.
Since most of the difference is in the magnets themselves not the wiring, I'm thinking just rewiring as the etrailer tech suggested won't in itself correct things. So I'm thinking of going with one of GlueGuy's suggestions and adding a dropping resistor to the drivers side circuit to see if balancing the currents between the two sides balances the braking action.
BTW, back on the original question on this thread, the etrailer tech also told me that the p2 max braking level adjustment should be near to the high end for a travel trailer and not to expect a TT to lock up brakes when setting the controller. He said that was more for a utility or car trailer in an lightly loaded condition.
He also said that the boost setting does not effect the max voltage output of the p2 but it does increase the output during light to medium range braking. He thought I should be at least at a boost level of b1 for my rig, and that would help a lot to keep the rear brakes of the TV from overheating on grades, and that b2 might be a better selection for long fairly shallow grades. b3 is reserved for rigs where the trailer weighs more than the TV.