would you trust a S.G. back wheel sitting on a chevy. tail gate for a 1k mile trip ,... I got a 6.5 bed chevy. truck ! 
I would be suffering from butt-hole pucker leading up to and through every good bump. That's a fair bit of mass raising up even ever so slightly and then coming back down on the bed straps and their pin mounts with each of those bumps. Even if the bike is strapped down well the weight transfer up and down is still putting energy against it.
Probably fine? Sure.... HPS (heavy pucker syndrome)? Absolutely. Motorcycles like long bed trucks.
Could you find or fab or long piece of C-channel that the front and rear wheel could rest in? That way all the weight transfer wouldn't be happening against the tailgate.