[Help] Single family member separation anxiety?

So I have a 1.5 year old half saint bernard/half unknown intact male who is generally a pretty great dog; he's gentle with our young daughter, never been aggressive in his life, for the most part well behaved (aside from becoming uncontrollably excited around other dogs/in the car, but that's a different issue mostly related to just his size/age), and very affectionate.Unfortunately, he has somewhat of an issue with separation anxiety, worst with the coming and going of mom. Basically sometimes when my girlfriend leaves him, he'll decide to take a dump in his kennel/the car and stomp in it. This is somewhat of an issue, being as how he's about 100 pounds so you can imagine how much shit this actually involves. If he's in the crate he can generally contain it to the back corner, but we basically can't take him anywhere in the car because he'll shit on the seats and walk around in it.This has never occurred with my coming and going, and only occurs when she leaves the house with or without our daughter. We initially thought this was due to him being more attached to her than me, but we've basically ruled out any form of trigger to cause this behavior; while I would say I am his primary caregiver at this point, she was previously taking care of him most of the time and he was still doing this. Furthermore, it's super inconsistent when he does this; he's done it three times in the last two days, but before that hadn't done it in over a few months, at which time he did it multiple days in a row as well. It does not seem to correlate with his activity level or anything like that; he's generally pretty low energy and well behaved with a short daily walk or trip to the dog park every few days.We recently asked our neighbor and I guess he quietly howls the entire time we're out of the house if he's left alone. He stays in his crate whenever we're not home and at night. We trialed leaving him out for about a month and he did well at night and whenever we left the house, but gradually started getting into things which culminated in him counter surfing, wrecking the trash, and turning one of my wiimotes into a chew toy. I've watched him with cameras when we're out of the house when he was not in the crate, and he mostly just looks out the windows and naps until we've been gone for longer than 1-2 hours, and then suddenly goes on a rampage.I'm not sure if there are environmental triggers in our apartment complex that are bothering him? The neighbor does have an unspayed female dog; could her going into heat be triggering some sort of rooting behavior that is causing him to become agitated? Overall we're kind of at wit's end as to what to do, since he's not really consistently doing this enough to rule it out what's going on. via /r/dogs http://ift.tt/2s21vnY