18 tons is probably 12 gears, possibly split axle (low first, high first, low second, etc)
the lgv test doesn’t cover the wave effect when carrying liquids – you’re driving along, having a jolly nice time, thinking about murdering prostitutes (according to Mr J Clarkson), when you have to brake suddenly. the truck slows, but the water doesn’t (inertial forces) so most of the water surges to the front of the tank, which counteracts the braking effect of the wheels, causing the vehicle to surge forward. But then the water, having hit the front of the tank surges backwward (wave effect), lifting weight from the front of the vehicle making it difficult to steer. Cue a jackknife event if the vehicle’s articulated, or fun and games if this is in busy city streets.
Not much a problem with modern equipment because most tanks today are fitted with baffle plates especially to stop this sort of thing from happening.
Have fun.