I've notice that my water tank under my raptor 5th wheel has a large bulge under it. They put a nice 1/4" plastic sheet under the whole rig and attach it with lags so I can't look under it without removing the whole sheet. Everything if flat except for the water tank, it droops a good 4". Is this the design of the tank? is this abnormal?

When you fill the tank, do you get air outta the vvent, right at the filler? If not, you may have to drop the tank down enought to see if the vent tube on the top is kinked or plugged. It's not too hard to do, if the tank is empty, that's a key rite there, empty!!

I've worked on a few fresh tanks that had the same problem, all were the vent tube.

Humm, not sure I follow. I put a hose in the filler and run it until it shoots out. Then I open the main drain line for a few to drop the level a little. The droop is there now and the tank is empty. So it may be that way from the factory for a low spot pickup or it may have taken a droopy shape.

Lots of weight and crappy supports, normal (well) they all do it when full.

Jackxclan has it right ..... I had to rebuild mine with thicker gage steel. I swear my sons bed frame was thicker......Easy to do but a must before you get cracks or falls on the ground..

On my first TH, a forrest river...I noticed my tank doing that. Coming home from Dumont, the little bugger decided to GTFO on the freeway. It was covered under warrenty but it pissed me off though.

Make the bottom straps into "T" angles. That'll make it plenty strong.
There is still a vent on top of the tank, it's connected to the water fill neck. There should be a small round screen area just above the fill hole. That should be open. If you fill the tank, water should come from that small hole first, then the big hole when ya pull the water hose out.

Oh boy, I can hear the comments now.

Bummer, I was hoping for a ; ya that's how it should be.