Toilets and travel

Hi all, just a quick question, is it ok to have water in the flush tank on a thetford toilet while traveling, I heard somewhere that it should not have water in it, I have looked in the archive, but not found anything to confirm my query, in the toilet booklet, in says not to have it full? Does this mean not to have it empty??? We are making a box to store the toilet on the draw bar of the trailer, so it has its own compartment. Thanks for any info. Cheers odog