The max voltage must be set, per the instructions, which are very similar (identical) to that of most aftermarket controllers. Some people here are confused, thinking that the integrated controllers set themselves automatically, and need no setup. That is incorrect. The OP (and others) are correct in that you need to set the max voltage, such that a full manual apply at 25mph will lock the brakes. Then you back off the voltage just until the same apply no longer locks the brakes.
Maximum brake torque is achieved with maximum voltage, but only as long as the brakes are not locked. As soon as the tires lock, they slide, and you lose braking torque. That's why such a condition could cause the trailer to jackknife, if the truck's tires are not locked. But as long as you have the trailer brakes set up properly, a maximum brake pedal apply will provide only the maximum set voltage (which you have set to almost, but not quite lock the tires).
As far as a stop on a slippery surface:
The truck's tires will start to skid, and ABS will kick in. You might think that this will cause a jackknife, but it won't. The brake system (whether integrated, or a good aftermarket like the Prodigy) will sense the reduction in deceleration of the ABS event, and therefore send back a reduced amount of voltage to the trailer. As long as the truck and trailer tires are in good shape, neither should lock up.
Some tips when setting up:
My Prodigy (and likely you integrated systems), call for first warming up the trailer brakes before setting up the voltage. Most people understand that extremely hot brakes can cause them to "fade" loosing effectiveness. But you may not know that dead-cold brakes do not perform quite as well as moderately warm brakes. Let's say you set them up cold, so that 7volts almost locks the brakes. After you head out and the brakes warm up, they will be more effective (more output for the same voltage). In that case, in a panic stop, 7 volts may lock up the trailer tires, causing a jackknife.
So, I like to do several gentle stops from 25-5mph. Have the windows open, and do 5 such "snubs". But if you start to smell the brakes, that's enough.
Now, if you've done all this, and you get up to maximum voltage and still can't lock the trailer tires, leave it at that maximum setting. Then, as soon as practical, have the trailer's drum brakes adjusted. Hopefully, the OP is certain that the trailer place actually did that.
If adjusting the brakes does still does not allow you to lock the brakes, then I would say the trailer manufacturers did not use the correct size brakes. This is unfortunately, but there is little you can do about it. Full size trailers are already using the largest brakes available to the market. The larger toy haulers get triple axles to handle the weight, and the extra 2 brakes for stopping power. Some trailers are in that gray area. Just keep it at the max voltage, and keep the brakes maintained every year.
thebrakeman ('70), DW ('71), DD ('99), DD ('01), DD ('05)
2004 Surveyor SV261T (UltraLite Bunkhouse Hybrid)
2006 Mercury Mountaineer V8 AWD Premier
Equal-i-zer WDH (10k), Prodigy Brake Controller