I suspect the tub is fiberglass. That material can be repaired with a kit available in some hardware stores, auto parts stores and marine stores, but making the patch invisible will take some work. If it's not fiberglass, I have no idea how it would be repaired. Tap Plastics is another source for the repair material and they mention fiberglass showers on their web site.
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.
while waiting for a reply, i found replacement tubs, and they were abs plastic. but your link had a repair for abs. thanks for sharing your knowledge. i probably should have mentioned i have an '86 gulfstream in case age matters.but again thanks for all your help.
Good luck with the repair (or replacement). Let us know how it turns out. Note that Lou had a good point regarding the possible lack of support that allowed the tub to be stressed and eventually to crack.
Some time in the 80's or early 90's, one of the ABS manufacturers had a huge problem that manifested itself in cracked drain pipes in homes. I have no idea if there's any difference between the ABS used for those drain pipes and the ABS used for shower tubs (other than color), nor do I know if this is in any way relevant. But thought I'd mention it since it might influence your repair/replace decision.