Karen, are you saying you have 70+% humidity from ground moisture? If so, the bath fans are a drop in the bucket; by all means correct them, but the ground moisture problem is the one to concentrate on (as you seem to be aware). Hopefully the Delta product will be able to cut down moisture diffusing through the concrete, but you also want to make your house stop sucking on the slab by lowering the neutral pressure plane down to the floor (the neutral pressure plane is a plane somewhere between the roof and floor where the outside pressure is equal to the inside pressure). That attic fan will significantly depressurize the house unless the ceiling is air sealed.
Do you know if the excessive moisture is due to a high water table or poor lot grading and or lack of gutters? What is it going to cost to cover the whole slab?