Just like ependydad says, they stabilize, they do NOT level.
You'll need a set of wheel chocks to lock the tires together to prevent rolling. (Even a little bit) Build or buy them.
Here's what I always did when I had a travel trailer:
1st, level it from side to side, using blocks. The giant "Lego" type that lock and stack together are ideal.
2nd, crank the front hitch jack 'til it's level, then go slightly low in the front end. (Maybe 2" or 2.5" for a 30' trailer) Whether you have built-in jacks or old "Christmas tree" jacks doesn't matter, put them in at the back end until they JUST TOUCH.
3rd, crank up the front end (hitch jack) until the trailer is slightly higher than level, about 1 to 1.5 inch. Same thing, make the jacks JUST TOUCH at the front end.
4th, drop the front hitch jack a few turns until the front jacks take a little pressure, and she should be solid with no twisted frame, jammed entry door etc.
Trying to level it with jacks can go so far as to break windows, wedge the door shut, etc.
On edit:
With wooden wheel chocks, try running in a series of 1/4" hex head screws to give the tires something to latch onto. Notice the metal ones you buy have "teeth" stamped in. The hex heads won't damage the tires, but should work like ice crampons.
And these days, all I do is run hydraulic jacks down 'til it's good 'n' level. They'll left the back tires up, even with a 17,000 lb. rear axle. :B
