Regarding the jacks in the middle of the trailer. We had a 98 Terry that moved too much when someone was walking around. I bought some cheap screw jacks, put one on each side near the middle, problem solved. I also used one under the steps to stabilize them.
On our current trailer, the frame is stronger and I have not needed the jacks in the middle or under the steps...
Find a method that works for you, most folks do side to side leveling first, then tongue jack for front to back, then stabilize with the scissor jacks on corners. Add a couple screw jacks in middle if the movement is too much for you.
Many folks just use blocks in front and behind tires to keep from rolling, but the wheel chocks that tighten between the tires add a level of stability. We made ours out of 4x4 and readi rod. Something like $10 for two sets.
... and as for the spot to use to check level, we like the frig to be level, the stove to be level and the bed. When we first got our trailer, we set up level and checked things. Once we were happy with the situation, we found a place that indicated level. It happens that just inside the door, the floor represents what we want for level. We have a small 6" level that we keep by the door and when are setting up, just use that for both directions. We have a slide on current trailer so we set up slightly high on the slide side and then when the slide goes out, we are level. I check the stabilizers and wheel chocks a few hours after setup or the next morning, occasionally find one a little loose.
Bottom line, find a simple method that works for you, is easy to repeat and have fun... forgot to mention, my wife sleeps later than I do, so it is important for us that when I get up and walk around fixing coffee, using head, etc that my movement does not wake her up.
