resilient cap 54 bears against the inner face of the trailer wall 16 and applies a sealing force to the gasket 22 appropriate spacing of the end 52 of the tube 35 above the top wall 34 of the slideout box 12 is provided by a standard in the form of a t bar 56 to the lower end of which is welded a cross piece 58 in use after the slideout box 12 has been moved into its inner position the brace member 28 is placed on top 34 of the slideout box 12 with the end cap 54 bearing against the inner face of the trailer wall 16 and with the flange bar 50 adjacent the inner face of the flange 32 on the slideout box 12 the crank is then turned withdrawing the lead screw 38 from the tube 35 and causing the bearing member 50 on the flange 48 to bear firmly against the flange 32 on the inner face of the slideout box 12 this action also presses the end 52 and cap 54 firmly against the wall 16 of the trailer and applies an inward force to the slideout box 12 thereby sealing the outer flange 24 of the box around the face of the opening 14 in the wall 16 by.