Technically you may consider this theft, but I imagine it to be life in the class B world. If YOU value those items don't leave them behind when you leave.
There will always be novice campers that will think some one left these behind and consider them free game.
If the neighbor camper really stole the items do you think he'd leave them in plain view! Do you think he'd be using your stolen cable?
No the neighbor believed the items were forgotten and considered them free game vs. sneaking over and stealing them.
I'm not justifying the neighbors actions, but I can see how and why it happened.
Many campers are simply not aware that the guy in the class B is just gone for the day and will return.
As a B owner I suggest you consider these novices and take you stuff with you at all times. Leave a sign or some non valuable item to stake claim of your site while you are away.
Yes they are taking your stuff but they mistakenly thought it was abandoned and claimed your items vs. actually stealing them in the traditional sense.