Clive,
If you have a decent low loader then mount a big Enduramaxx tank or similar on that, main thing is keep the centre of gravity low as possible.We use a plastic tank with about two hundred baffle balls inside to stop it pushing around the grain trailer its bolted to inside.
Thinking about it, a decent grain trailer has everything you need, sprung drawbar, wide tyres ALB etc, the tanks can come with extra long subframes to mount Honda pumps etc onto.You also have plenty of safe area for ag chems etc.
Our 8000 litre cost around £3250, thats cheap bowsering , and the tank is well protected from side impact etc. No guarantees for rollovers though!
At harvest the trailer is put back to grain spec.