Eric, Your idea of the tank may indeed be the reason or at least one factor in the equation. I do think the Airhawk will give you 2 inches or so height and that may help.
Do you have a backrest on your seat? If not, that too may be of benefit. I installed one this spring and it has helped both my back and hip comfort quite well by forcing me into better posture.
If you are able to, maybe some exercise will help as well just to keep you limber. Squats, stairs, yoga and stuff like that.
I hope you figure it out brother.