Will motion sickness prevent me being a tank crewman

If it is so serious that it is on your medical records then the answer is probably yes. If you get motion sickness then personally i'd ask yourself whether that's really the role that you'd like to go in to. There are people that get motion sickness in a tank even if they don't usually get motion sickness!!! So, if you know you do then i'd really consider something else. Also given the close proximity you are to each other in a tank I'd be pretty annoyed if someone was sat next to me being sick especially when you could be in it for hours! It's not like a car; just pull over, sort yourself out and get back in!