If the tub is made out of plastic, it's probably a Duo-Form product.? The real problem is the support under the tub isn't sufficient to keep the plastic from flexing as you walk around while taking a shower.?Duo-form puts about 2" of high impact styrofoam under the floor of their tubs and showers - it looks like a piece of left-over packing material. Unfortunately, it tends to break down and compresses under heavy use.? ?Once it compresses, the plastic flexes and you develop a stress crack.
You can use a fiberglass patch kit to fill the crack, but the real solution is to eliminate the source of the stress that fatigued the plastic in the first place.? ?Remove the tub and fix the support as needed.? ?If you're going to do this, you might as well put in a new enclosure.? ?They're available through the Coast Connection catalog (check with almost any independent RV service shop) or maybe from your rig's manufacturer.
If you decide to patch the crack, drill about an 1/8" hole just beyond each end of the crack.? This will relieve the stress and help prevent the crack from spreading.? ?The same technique works when repairing cracked holding tanks.