Is the crack on the shower pan (where you stand), or on the wall (the shower surround)?
Either way, you should drill a small hole at the end of the crack (each end and any other splinter cracks that might have occurred. This will keep the crack from splintering farther.
To repair the best, you should remove, the pan and, like suggested above, apply a fiberglass resin filler (if on the shower pan ... floor) both sides, underneath and on the top. Once dried, sand down to make it smooth.
If on the wall, you need to do the same thing, drill a small hole at each end of the crack to keep it from spreading. Then fill with a resin epoxy, or tape.
If the crack is on the surround (wall), you really need to drill out the screw holes where it attaches to the walls so the surround has room to wiggle. More than likely, when installed, they drilled a small hole in the surround and then inserted a screw and attached to the wall. Bad, bad, bad way of doing it. There is no "give" in the surround. The slightest bit of stress or the slightest temperature changes causing the plastic-fiberglass to expand or shrink at a different rate than the wood behind it, will cause stress on the screw, causing stress on the surround, causing a break. So, make sure the holes drilled in the surround and the shower pan are larger than the screws so there is some wiggle room when the camper flexes or temperatures change.
