First, have bought two new trailers, '91 HH 40' and '98 38' HH, tank size was an issue so that's why one reason we purchased these units. With as many options(trailer choices)look at another model/brand that meets your needs.
Usually you can see where the drain lines go into the undercarriage and that's where the tank(s) will be. Personally, I would not consider a unit that I would have to replace a tank after purchase. Look at web site of other units, I would almost guarantee you will find something you don't have to re-engineer.
Jim