I went with Plastic Mart when I was replacing my holding tank http://www.plastic-mart.com/ but I was recently talking to a guy I work with that has just finished building a Bruce Roberts designed 37 footer. He looked around at tank options and, like you, wanted to maximize the tankage for the available space. He looked at Ronco and others but ended up going to a local plastic shop (same shop I just bought the acrylic for my new fixed ports) and had them make custom tanks for him out of x-linked poly. They had RF welding equipment and did a very nice job for him with fittings and baffles installed. The local company here is Canus Plastic http://www.canusplastics.com/ but you may have a shop near you that does the same. They have a big business in signs and display cabinets so you could search under that near you.