I store the same way...18' wide by 40' long open on both ends metal shed. You can see the little holes in the ground where they buzz around, loading up dirt. Spray the ground under the shed and MH with Cypermethrin diluted down per instruction. You can also spray the bottom areas of MH and around the vents, domes, etc. Another product that works is Pyrethrin. It works great for other flying insects but not as good on daubers as Cypermethrin. I mix Pyrethrin with diesel and use it in a LP powered fogger to treat the area you are camping in...no flies or mosquitoes for hours. Is the ground under your shed dirt or paved?
2018 THOR Chateau 35SF
Two Labs, Bugsie & Blondie