www.rvecafe.com/tank.html
This site may help ?

Here is a whole page of possible sites.

search.yahoo.com/search

we used JB weld..

and we have extra fiberglass and resin if you need it..

Fiberglass isn't difficult to patch. The crack is right at a joint between the fitting and the tank, which can complicate things, but since appearance is not important you can lay on as much material as you'd like. I definitely think you should fix this before BM.

You can Google about fiberglass patching, but essentially it's a process of layering resin with the fiberglass weave. I think you're looking at an hour or so, and about $30 in material.

if you remove the valve, you can also get an extra layer of glass matting on the inside of the crack, just to be extra certain of making a good fix of it.....like Big Mike said....it's neither an expensive or difficult repair......just make sure that you roughen up the area where the new matting will be placed to get it well and truly 'keyed' in for a good bond.

I've never posted here before (but have lurked a lot). I went ahead and signed up to post this, hopefully it'll help.

I just had the exact same issue, in the exact same spot, but on my grey water tank instead. Several nasty splits, right next to the pipe. After searching some of the forums, I went with a 2-pronged approach: JB makes a product called WaterWeld, which is an epoxy putty. I drilled a small hole in both ends of each crack to keep it from splitting further, then applied a thin strip of the JB weld. After giving it about an hour to cure properly, I then covered the whole seam with the eternabond tape.

Right now, I'm waiting on a replacement pump for the fresh water tank which should show up tomorrow or the next day. I'll post back to let everyone know how the method works. I can't remember how much the tape was, but the WaterWeld was $5 (mine took 2 tubes).

Depending on how this goes, I may just replace the tank outright next year (it had a LOT of cracks), but if I can stop the leaks for this year, I'll be happy with that for the time being.

Looks like the method worked great! However, the tank is in worse shape than I thought. The original cracks were on the bottom of the tank. Now that they're holding water, the level is rising, and has exposed other cracks further up the tank. Whenever I fix one, the rising water exposes another (I'm up to three so far). If I can't get this completely bone-dry in a few more days, I'm gonna leave it as-is and run a grey-water evap pool under it and extending out from the vehicle (we've had good luck in the past with black plastic stapled over a 2x4 frame), since it's the grey water and not the black, so no stinky. Best of luck on yours, and look up TIKIPLEX on the playa. We'll give you Rum and questionable cinema.

i don't think that your tank is fiberglass. if it is fiberglass it is very easy to repair with a fiberglass repair kit. the kits are at most auto parts stores. if your tank is black in color, it is made of ABS plastic. most epoxy & glues won't stick for very long. but ABS can be repaired easily & permanently. buy a pint can of MEK solvent (home depot has it) & some black ABS scrap pipe. file or saw the scrap pipe into a coarse product. drill small holes in each end of the crack. clean the area very good & use a wire brush & brush it well. using a small can (not plastic as it will melt)
pour in a couple of ounces of MEK & add a small amount of ABS filings & stir until it becomes a very lite syrup. 1 st.- using a cheap 1/2" brush,paint the crack with full strength MEK .lit it dry for a few minutes & repeat. again let it dry a few minutes. start applying the ABS & MEK mixture. let it dry between coats. it may take a while between coats so cover the top of the can so it doesn't evaproate. if your mixture gets to thick,add more MEK & stir. a thinner mixture is better than a too thick of one & will dry faster. the tank must be DRY. leave the dump valve open for a few days before repair is made. this will work on the black abs tanks only. the white plastic tanks must be welded with a plastic welder available at harbor freight. put skid wheels on the back bumper so this doesn't happen again. harbor freight also has large casters for skid wheels. hope this helps ,i've use this many time so i know it works well.will cost under $10.00. al