Most larger trailers, like you have, will not lock up the wheels if you are on pavement. If you try it on sand or gravel you would probably get a different result. There is just too much weight and friction on the tires for them to lock up by the brakes on a travel trailer.
This is not to say that some people cannot get theirs to lock up. Some folks on here have reported that they can but I have never been able to, even when I just finish adjusting my brakes. This has been the result posted by most folks on the forums. If you can feel a strong pull back by the trailer when using the manual lever then I would not worry about it. I would, however, give the brakes an adjustment to make sure they are working correctly.
Barney