The sagging tank is one issue. Possibly an issue, possibly not.
The side outlet tap is a separate issue. Bad design if your goal is the ability to use all the water you've carried. My last trailer had the side tap. It also had a drain tap in the bottom. So I replumbed it to where the pump pulled from the bottom. Tee'd in so that I still had a drain as well. Figure I gained at least 5-7 gallons of capacity.
My current trailer has not only a bottom pickup, but the pickup is in a sump area, so it can pull 100% of the water. Much better design, but perhaps more likely to pickup debris that is found in all water systems. Keep an eye on the pump inlet filter.