the classic and safari lines have different frames.
the tanks rest inside the frame rails and cross members.
i'm sure it's possible to put a larger (deeper) fresh tank in place of the current tank,
but it will take effort and $$ just to gain 10-15 gallons...
it might be easier to add fresh water capacity in the t.v.
a variety of bladders and auxiliary tanks with pumps are used.
fresh capacity can be 100s of gallons using this approach.
here is just one thread on this topic.