You said the last 2 Sunlines, what was the other TT size?
I can add some to what you are seeing as mine will not lock either at medium speed wiht the manul lever even at full ppwer. It comes from a few areas. The bigger the camper the worse this may get.
1. Brake controller not givning enough power. This is a cause but I think you already adjusted past this one so in your case this is not and issue I think.
2. Getting full power to all 4 wheels. You may or may not be reaching full amperage at all 4 TT wheels. A rusty ground, TV 7 wire connection and small gage wire are issues sometimes. On mine I upgraded the wire. Sunline at least ran number 10 from the front to the back. Some brands do not even do that. However once at the back they split into 14 awg feeders going thru the axle tubes and then there is the splice at the ends. Each splice can loose a tad of voltage drop and then the wire is marginal.
I upgraded to no 10 cross wires, outside the axle tube, and a no 10 ground. Each brake is totally independent of each other and all joints soldered, shrunk wrapped and taped. This helped get full power to the wheels thru the brake controller.
3. Next is the brake itself. Are they all adjusted dead on and up tight where they are suppose to be? I’m assuming you have the std TT/5er brakes that are not self adjusting. If you do no crawl under and tweak them about every 3,000 miles they loose braking strength or you keep up’ing the brake controller to compensate. Well full power may not mean full braking if they are out of adjustment. And all 4 the same. Do you adjust your own?
To help solve that problem I upgraded to self adjusting ones. Looks like this.
The old standard ones 12 x 2 brakes
The new self adjuster Dexter brakes 12 x 2 brakes
This gets what you have about as good as it is going to get. The weight and size of the camper along with the speed can also change if you can lock the brakes. Dexter states in there application guide and manuals that brakes will not lock when close to full load on all sizes. They will brake within acceptable parameters.
A 10,000# flat bed trailer loaded to 4,000# will have a good chance of lock up. A 10,000# GVWR TT loaded to 9,200# is not going to lock with 12 x 2 drum brakes at much above 20 mph. They will brake real hard they will not lock and they are not oversized enough to hold the truck too. Id the truck weighs 8500# and the TT 9,200# that is 17,700# trying to stop. No way are they strong enough to stop that kind of load. They are only sized to the GVWR of the TT. The truck stop the truck, the TT stops the TT.
In my case what drove me to those upgrades was a few things. Not having self adjusting brakes in the year 2009 is truly a miss on the part of the RV industry. There is only about $5 worth of parts when building new to make them self adjust. We are riding around on 1950 brake technology.
Next was the brittleness in the wire inside the axle tube. It can crack and short that one brake intermittingly. My wire was so brittle I flexed it about 30 degrees and it cracked in my hand. So out it went and I can get better amperage at the wheels now.
My F350 has the Ford Tow Command integrated brake controller. I know my truck was doing more stopping as the rear wheels dusted like heck when I towed. The Tow Command is so sensitive that the TT will not lead or lag the truck, but the truck will do more stopping. Ever since upgrading the dusting went away. Having your brakes adjusted up to snuff all the time makes a difference.
You may not be able to ever lock yours doing more then 30 MPH but you can make them the best they can be.
Hope this helps