Thanks all for the help. I just wanted to update the thread with my action and results.
I was able to weld the crack without any issues using the cheap harbor freight plastic welder (large soldering iron looking thing). Having no previous experience with it, I practiced on some other plastic I had laying about the garage. If I have to do again, I will pay extra attention to make sure that I have sufficiently melted the tank plastic for a better bond. The tank is Medium Density and my welding rods were Light Density. After one trip, post repair, the weld is holding.
I was able to support the bottom of my fresh water tanks in the same manner as mhamershock. I fastened a piece of 3" x 3" angle iron the leaf spring mounting brackets. I then ran a 1" x 3" length of tube steel under each of my fresh water tank, supported by the angle iron. I was fortunate with the layout of my leaf spring brackets with respect to the location of the fresh water tanks. The tanks do not sag anymore so I not longer concerned about fashioning a lower drain valve.
Final note. Neither support is directly under the center of the fresh water tanks for a couple of different reasons. As a result, I will be watchful to see if the current support creates a stress issue elsewhere. If I do run into an issue with the current repair, I will likely utilize a longer length of the 3" x 3" angle iron and then run two steel tube supports under each tank, twelve to eighteen inches apart.
Cheers!
Dan
2005 Ford F-250 Fx4 Longbed
2007 26' FSAK Eclipse Attitude Toyhauler