Same thing with ours, it bulged down on either side of a single channel iron support. After inspecting the support I realized it was held in place by self threading bolts into the frame. What if they failed!! I purchased two pieces of 3 inch angle iron and bolted them on either side of the factory support with grade 5 bolts with lock washers and nuts. Now the tank does not sag even when full.
It seems all trailers are now made quickly with little thought as to how they will hold up. Just get them out the door, get the check and good luck to all. Also since weight has become a large consideration in today's products, the extra pounds from extra iron would be unacceptable to a designer.