Full of water, Id be interested to see the results, I wouldnt want to be the one driving it though.
I spent £1,000 on my van mount, for the tank and gubbins and fitting it. If it wasnt fixed to the vehicles chassis and I was in an accident what would be the point of me saving the £1,000 by fitting it myself if I was dead, I wouldnt be able to spend it then anyway.
Id rather be alive and £1,000 worse off than dead and flush.