We have a 40 footer on a seasonal site also.
First level the trailer side to side by the tires. Then you can level it with the front trailer hitch. Then place blocks in at least 4 places on each side, and use a hard wood wedge to tighten the blocks to the trailer frame. you should be able to get some of these wedges from your local dealer. Double check the level at the various areas. We used a bottle jack to fine tune each area, and it worked out great.
I hope this helps
Seasonal in Wi Dells, WI
2007, Salem 4022B