At each end of the slide, screwed to the living room floor, there should be a small metal plate with a hole in it. This plate becomes visible when the slide is pulled out "just enough". There is a matching latch pin (red handle) on the end of the slide just above the hole. This pin should drop down into the hole, to latch the slide in place. This is what defines "far enough".
So why does your slide go out too far? There are two wooden stop blocks screwed into the floor of the trailer, one on each side, just an inch or so back from the very front edge of the floor. They are not normally visible, but if you go outside the trailer, pull out the slide, and lift the front edge of the slide, you can see them. It sounds like these blocks are missing in your trailer (someone slammed the slide out so hard that they dislodged the blocks), or they have become rounded to the point that the slide simply rolls over them. Either way, they are easy to replace if you lift and support the front edge of the slide.
You may find some helpful info in this very old thread
http://www.trailmanorowners.com/foru...read.php?t=340
No, your slide should not hit the jack stem.
Bill