I replaced the black tank and retro-fitted two grey tanks in my '73 Globetrotter. The black tank was ordered from INCA Plastics, which was the original manufacturer of the tank and still has the molds. So I was able to get the exact tank I needed.
The grey tanks, I ordered from Vintage Trailer Supply. At the time I did mine, they only had grey tanks designed to fit in the frame rails of the 60's vintage trailers, so that is what I used. Today, I believe you can get slightly larger ones optimized for the slightly deeper frames of the 70's trailers--this is what I would recommend. I opted to put my tanks in the first two "bays" in the frame behind the axles, which I would still recommend. This leaves the aft-most bay open for the dump valves and drain pipes. My tank supports are modeled after the support for the fresh water tank, ie., I welded an angle-iron frame onto the underside of the trailer that allows a 1" thick plywood panel to slide in under the tank. I hear about lots of people supporting tanks with just ~3 straps going from one cross-member to another, but that seemed like a rather flimsy way to go.
Good luck!
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