Maiden Voyage with Trailer Brake issues!

[1966burb]

I have been working on the GHOST (66Burb) for about one year getting ready for camping season this year. Thought I had everything in order for the first outing. New brakes, wheel cylinders, hoses, new master cylinder and it has factory Power Brake unit. Shasta trailer weighs 2820 loaded and has a 360 # tongue weight. Single axle trailer has electric brakes which I went through and they work fine. New electronic brake controller. I can manually lock up the trailer brakes. I came down a 9 percent grade which was about 1 1 /2 miles and was assisting manually with trailer brakes. I was completely out of suburban brakes when I hit the bottom. They cooled off later and worked fine on the trip home. (avoided the hill) Pretty scary!! Any ideas? Or time to gear up and add power disc brakes? I was hoping to keep the burb somewhat stock. Will disc brakes work with the stock 15 x 5.5 inch wheels I am planning to put back on?