You had the right idea. If you only welded the top surface and aren't absolutely positive about full penetration of your weld, then you need to keep a close eye on it. It could rupture when full and traveling home.
The best way to repair that kind of damage is with a plastic welding kit. A large repair like that tank might work best with the kind that is like a modified heat gun. I have the kind of welder that looks like a giant, really hot soldering iron.
The actual repair would involve grinding the crack into a V so you can get full penetration. Then fill the bead. Finish by welding a strip of steel mesh onto the whole length of the crack and cover with a thick/wide layer of abs to make a super solid weld. I would trust that process to make a weld that lasts the life of the coach.
Jose
