The replacements for the 711 M28 that will fit under the folding shower are the following:
https://marinesan.com/5000-m65-serie...white-or-bone/
https://marinesan.com/700-m65-series...t--white-only/
Doesn't look like the tank comes with the 3" bottom discharge so you'd have install a 3" bulkhead of some type instead of using the pump out. At least you can put it where you want.
https://www.amazon.com/TF300-Polypro...31981818&psc=1