First I wouldnt try to level a trailer on grass. Put the tires on some boards or the links leveler blocks.
Level the trailer, CHOCK the tires, I use the BAL Chocks that go between the tires, and they really lock it in one place. Once the trailer is level and wheels chocked, slide out the slides, and DONT WORRY ABOUT ANYTHING ELSE. The trailer needs to be level for the slides to work and seal accuratly. It wont change enough to worry about after they are out.
" Let us endeavor so to live, that when we come to die, even the undertaker will be sorry." Mark Twain