Bottle jacks may not go low enough for what your want. They also have a very small head at the top of the shaft. This is normally OK for changing a tire, but when lowering a whole trailer, the trailer could become off-tilt when lowering it a long ways causing the bottle jack to slip. I think I'd use floor jacks with the larger contact surface if you have them.
I assume the trailer has two axles? If so, I think I'd work from side to side lowering it.
As for the blocking, treated lumber should be used for the ground contact boards, but rest of them don't really need to be treated if you don't have them. (Watch out for slivers when using treated lumber, they fester very easily.) If the trailer is being lowered on dirt and not cement, then on the bottom layer, I'd make it two 2X6s wide for a 12X12 pad and make the "bottom pad" two layers thick - in other words the bottom two layers should be 12X12. Also take some smaller / thinner pieces with you as you'll have to do some shimming to get everything level. If you store the tires there, put them on some wood also.
Bill
Nodwell RN110 out moose hunting. 4-53 Detroit, Clark 5 spd, 40" wide tracks, 10:00x20 tires, 16,000# capacity, 22,000# weight. You know the mud is getting deep when it's coming in the doors.
