I read in my toilet instruction book that you shouldn't travel with the flush tank full. I thought this was a bit strange, can anyone please help out on this?

Sounds strange to me as well... so what are you sposed to do when nature calls "en route", stop fill tank, "go" flush and empty tank???
I think from my vast experience using chemical toot's you would be quite safe to travel with your "top" tank full....
Maybe storing it say over winter in  a place where the contents may freeze would reqiure that it be emptied and cleaned. I found that the best way to clean these things throughly is to put in a squirt of Domestos ( or the ALDI equivalent) fill with warm water, slosh around for a minute or so, leave stand for say 15 mins then empty all down sewer....
Your toot ends up being so fresh and clean it's a shame to do a No2 in it....

I always leave the toilet top tank with no more than half in when travelling and when storing the van. The reason for this is one time I stored the van with a near full top tank and the heat in the closed up van expanded the liquid and forced it up through the pump and filled the bowl before overflowing and soaking the floor.

I have been travelling for the past 3 years, and have never experienced any problem with the flush tank being full when travelling.  I checked my book on the toilet in my van (Thetford brand) and it does not mention anything about the concern you have.

Thanks all for the advice, had pretty much decided to ignore anyway.
Checked the book to make sure I had read it correctly and I had. Mine is also a Thetford brand

Cheers