11 gage is a somewhat thin plate, so chances are that you'll have some warpage even if precautions are taken. So, the question, in my opinion, is not how to avoid warpage but how to minimize it. When positioning the parts before welding them, use the fixtures which are normally employed to hold the plates in position. These fixtures are generally called "strongbacks" and they are made in a lot of models. Choose the ones that are better for your case. For welding, use the "backstep" method. The plate is quite thin, are you sure you need to weld it from both sides? If so, don't make a whole pass on one side before making the other on the opposite side. Rather, make a portion of one pass on one side and balance it with a portion on the other side until you've completed the weld.
Once the tank is completely welded , chances are that it shows some distortion. Don't get in panic, this is normal in thin plates. You'll have to hammer the distortions from the appropriate side until they are strighten out. A word of caution: as the tanks are stainless steel, don't use carbon steel hammers. As there are no stainless steel hammers (at least I don't know them) either you make one from stainless plate or line a common hammer with a layer of stainless steel.
Giovanni S. Crisi
Sao Paulo - Brazil