IIRC I paid about $250 a tank. I purchased two 26 gallon "T" shaped tanks that I could hang from angle irons between the frame rails. Shipping the bulky tanks was expensive as you can imagine, but they were lightweight enough for UPS ground.
I thumbed through their PDF catalog until I found the tanks that would fit my Trade Wind. These tanks were only 17" wide and that would fit in the "narrow" frame bay beneath my selected toilet placement.
I recommend you find the tank that will fit your plans and then call them to place your order. That's what I did.
By the way, installing these waste water tanks was a BIG project for me. Belly pan and old insulation down, replumb the drain lines from bath sink, shower, kitchen sink, new toilet relocated to a spot between the frame rails, hang the tanks, determine the locations where the vents and drains enter the tanks, install SeeLevel senders, do the final assembly, install new Valterra drain valves and then test for leaks, and finally build a tank cover and run heat to the "tank compartment". Big, really big. Thank God the system works as planned.