as with everything, "it depends"....on your budget, among other things. Yes, you can get a custom tank to fit that area; it'll cost 2-3x what a "stock" tank costs, and still won't be all that big. well...again, that depends on just how much room there is in your trailer; in mine, the x-members in the frame are equally spaced. in others, they are not. Some people have re-located x-members to accommodate a larger tank; that wouldn't be practical with my particular trailer.
My personal opinion: buy a stock tank, big enough to be worthwhile (another subjective judgement. mpo: less than 25g="why bother?" ), and let it hang below the frame, cut a new belly pan to fit. (see RJ's restoration at Vintage Airstream Home
for an example). Nobody's going to see it under there, anyway.
I may do this, but stack a 3" c channel below the existing c-channel frame (which is essentially how sub-floor tanks are supported on old and new trailers), creating an 8" deep space. then cover it up w/ new belly pan, enclosing everything. it won't look original, but it'll still look "factory". or "factory-like".
and of course, as Andy says, beef up the frame-floor-shell attachment as appropriate to handle the extra weight.