Any WD hitch with chain links would be easy to adjust accordingly by changing the number of links used. I doubt the full tanks would increase the tongue weight that much that you would have any noticeable problem with your truck though.
A full 40 gallon tank would weigh approximately 320lbs and likely less than 1/2 of that would be applied to the tongue. Are you experiencing sway or bouncing or control issues? If not I'd stick with what you have. Adjust the hitch for the full tank weight and it will be fine if it's lighter.