I think the spring action of the tie-down will be very important:
This picture was shared by Chris Scott in a thread in his Horizons Unlimited overland travel forum relating to the building of camper bodies on flatbed trucks. I don't think you will be facing this extent of chassis twist, but you are right to consider it and allow for some twist in your tiedown design now rather than end up with structural damage to your newly built camper.
FYI this looks like a British Army Bedford MK someone is converting. I've driven these offroad and they are designed to flex.
Steve.
