Pull the drain valves
Also trailer air usually has manual controls on it...
Draining tanks doesn't empty the bags,, all trucks on air have a ferry setting which dumps air from unit, trailer would use lowering valve.
And how long have you got?
Some old Volvo's we used (or might of been Scanias) had a ferry setting but few knew it was there and nobody I know ever used it.
All current air suspension units can have the back end lowered or raised from inside, but it just isnt done IME.
Maybe that's why this thread got started. In any case I'm sure the lashings are there more for positional stability rather than stopping them rolling, which in 'normal' storm conditions I guess is more likely caused by unstable loading.
Doesn't bother me anymore though. Some bright spark realised there was a constant stream of paddy wagons running home empty from the continent and literally passing our door, so they get a backload and I had to find something else to do.