Beam clamps are available at Lowe’s or Home Depot along with uninstrut channel. I would sandwich a section of coroplast between the sagging tank and at least 2 equally spaced pieces of unistrut between the frame rails. You could fasten the unistrut to the frame rails with unistrut beam clamps( may vibrate loose on a trailer) , or drill and bolt the unistrut to the I beam flange. The coroplast between the tank and unistrut would insulate the tank from chafing on the unistrut (unlikely because unistrut is smooth with radius corners), and if you cut and install the coroplast it as a separate section (overlap the edges) it will be easier to service the tank later if needed. It should install relatively easy with empty tanks.
After posting, I re-read your first post to make sure that I understood. If insulating the tanks is the objective you could still use the unistrut to support the sagging tank, and sandwich closed cell foam board between the tank and the coroplast. You would just need longer bolts to fasten the unistrut to the i beam flanges (beam clamps would not work in this case). Also, this would be a good opportunity to install an electric tank heater pad.
