The scottish wild camping access code (legal tent camping ) says this
" The Access Code says:
If you need to urinate, do so at least 30 metres from open water or rivers and streams. If you need to defecate, do so as far away as possible from buildings, from open water or rivers and streams, and from any farm animals. Bury faeces in a shallow hole and replace the turf."
We do a lot of fast packing running in the mountains with ultra light camping kit. We carry a 29gram aluminium trowel, made from a special snow camping tent peg.
Snow camping is the problem as you might have to dig through a lot of snow and ice to get to ground that will not melt to bury!
In the camper in suitable rural areas we follow this advice but use a larger ice axe. Once faeces are buried off paths and stream courses 150 to 200mm deep leaving little trace they cause no problem. They rot quickly, quicker than the toilet paper.