I go to Wally and buy a bucket called "DAMP RID" it is in the RV accessories aisle and (the one I bought yesterday) cost $9.88. All you do is remove the lid and place on floor. I leave it in all year, and replace it every year at winterize time. You can replace the lid when using the camper, but I forgotten to at times with no negative results.
It is calcium chloride (fragrance free) Usually by the end of summer I end up with a very full bucket of partially crystals and partially water.
It does work very well in my PUP trailer. Everything ia always dry when we get back in it. And the canvas is dry with no mold or stains.