This has been a topic that has been discussed quite a bit lately on the forum. You can catch up on that discussion here:
http://www.jaycoowners.com/forums/f3...ons-37507.html
Much of that discussion has centered around the performance of Factory Brake controllers - and I know you have the P3 (an excellent controller by the way).
Here's my opinion - you may get others here:
1) Yes, You should be able to lock up your brakes - and you'll know when you do, lots of squealing and smoke just like when you lock up car brakes.
2) Sadly you can't always trust the dealer, or rather a dealers employees. Some of them lie but most of them just simply don't know what they are talking about. You have to do your own research and this is a good place to start.
3) Several members, including me, as you will read near the end of the thread above, have discovered that it takes quite a while for electric, self adjusting trailer brakes to actually "settle in" and self adjust enough.
I used to pull a 20ft Sunline TT with a P3. I couldn't run it past a setting of 3 or the brakes would lock up on even a soft stop. And I always felt like I could make the trailer stop both the truck and trailer.
Read the thread I referenced above and If your's don't get better after a few trips - take it back to the dealer and insist they pull the wheels, check that all magnets are getting power, that the brakes are in adjustment and that there is no grease on the brake shoes. Those are the three most common problems for poor electric brakes.