i started my restoration a few months ago a weekend at a time.
i too had the black water tank sagging in the rear. i put a temporary bandage on it by placing 2" x 1/8" aluminum strips & drilling self tapping screws to the bottom of the c-channel frame and it the width of the frame. you can get these alum. strips at lowes or home dpot along with the self tapping stainles screws. upon removing the subfloor and gutting the back end i found that the steel that was supposed to hold the black water tank in place was rotted out. see pic attached.
i have cut the existing channel,angle steel out and replaced the cross members and out riggers that you can find online at
www.outofdoorsmart.com
i have replaced the steel around the black water tank and am now getting ready to put the flooring and the plumbing back in. i have dimensioned drawings and parts lists with dollar amts. i thought of replacing the floor with this cutting board type material in the web link below but because of shipping costs i'm going back in with pressure treated plywood and just applying thompsons water seal to it.
Cart - ePlastics - Ridout Plastics
i can send you all dimension drawings later if u need them have a good day. i have attached a snap shot of the steel that i replaced in a photo and a 3d model as well.