Never had any problems in UK or abroad, always just use whatever's to hand (tree, fence, lamppost etc.) with a d-lock and have even just left the bike on the ground next to the tent, locked through both wheels and frame with d-lock and cable.
If you were particularly concerned about a place looking/feeling dodgy and there wasn't a tree nearby, I'd just ask the campsite owners if there's a shed/garage/somewhere a bit more secure you can lock it.
I have heard of people tying guy ropes to the bike or even a thin cord from bike to body (fingers/toes) to ensure any tampering awakes the owner but it seems a little over the top to me.
If you were particularly concerned about a place looking/feeling dodgy and there wasn't a tree nearby, I'd just ask the campsite owners if there's a shed/garage/somewhere a bit more secure you can lock it.
I have heard of people tying guy ropes to the bike or even a thin cord from bike to body (fingers/toes) to ensure any tampering awakes the owner but it seems a little over the top to me.