You might want to check this repair kit:
https://www.etrailer.com/RV-Fresh-Wa...SABEgIaG_D_BwE
If your tank is ABS, the above is what you'll need. If your tank is not ABS, then this should work to repair the split:
http://www.truevalue.com//catalog/pr...64-49748505579
Another alternative is a "fitting relocation kit". You'd need to use the kit's plug to seal the existing leaking hole and install a new drain fitting from the kit:
http://www.plastic-mart.com/product/...-tanks-90-2218