its a 90 gal metal tank with a split in the bottom
Most pro welders will not touch a fuel tank after it has been used no matter how well you clean it. Filling it with water then welding it is the safest bet but it is still a gamble. Do yourself a favor and try to get a replacement from a scrapyard. Please do not learn the hard way about welding on fuel tanks.
If the split isn't too big, JB Waterweld may be able to fix it.They sell it at Wal-Mart.Here's a company link to the product:http://jbweld.net/products/water.php
You can probably find a suitable replacement in a junk yard. Just call ahead and see if they have any motor homes.
If your going to weld it better do it full of water to remove chance of fumes exploding.Or better get a pro to do it.might be cheaper to source a second hand one from fleabay.
Remove the take and clean it out really good so there is no gas of fumes left in it then weld the split. Thats only if replacing it isn't an option.