Dry Fit the Fifth Wheel
Position the fifth wheel into the bed precisely as it will be when installed. If you will be drilling new holes into the bed, pre-drill the holes for the fifth wheel through the bed liner and dry fit the fifth wheel into place, but do not fully secure any bolts or hardware associated with it.
Trace the location of the fifth wheel onto the bed liner using a white or light-colored paint marker and remove the fifth wheel. Remove the bed liner from the truck and place on a stable surface. Support the area that will be cut with wooden blocks and clamps as needed.
Check the outline a second time with the liner off of the truck to guarantee the shape of the outline is accurate.
Cut and Install the Bed Liner
Place masking tape or painters tape along the outline of the fifth wheel. Place a second piece over the outline. The lower piece of tape will leave an outline in the upper piece. Trace this line with a pencil for easy reference.
Cut out the shape of the fifth wheel using an oscillating tool. Popular Mechanics labelled the Fein MultiMaster 250Q as the best oscillating cutting tool. However, you can use an angle grinder to make the cuts. Be careful while cutting, as the section that you cut out will be used as a template to make a rubber gasket. Slowly cut along the pencil line until you have completed the cut.
Sand the cut edges of the bed liner. Cut plastic can be sharp. Use fine-grit sandpaper to smooth out the edges of the cut.
Trace out the shape of the holes onto the rubber gasket material using the template you made when you cut the bed liner. Use the white paint marker to trace the template. If the cut-out plastic was damaged, use the bed liner holes as the template.
Cut out the holes without cutting from the edges. Use scissors and start in the middle of the traced lines instead of on the edge of the gasket material. You want a solid ring of rubber without any breaks in it to place under the bed liner so that water will not get under it. If you have to use multiple sheets of gasket material to make the shape work, make sure that the pieces overlap giving you two layers where the sheets meet instead of a gap.
Punch holes around the gasket. The holes will allow bolts to go through without tearing up the gasket material when you reinstall the bed liner. You only need two or three per side. Place the gasket centered over the holes in the bed liner and trace those holes so that you know where to drill when you install the liner.
Place the gasket liners in place on the bed of the truck. For multiple-piece gaskets, overlap the edges and apply rubber cement to the two pieces so that they bond together. Install the bed liner. Make sure the gaskets are positioned properly between the liner and the bed.
Drill from the points marked on the liner through the liner and bed. Install bolts through the holes and tighten the nuts from the bottom of the bed. Make sure there are no gaps between the liner, gasket and bed. If any gaps do exist, loosen the bolts and shift the liner or gasket around to compensate. Use small pieces of the gasket material as needed to fill some holes. Make sure you apply rubber cement to any piece used so that it bonds with the other gasket material.
Install the fifth wheel per manufacturer directions.