Mental or emotional illness is not a license to rude. There is no reason for you to have to walk on egg shells with your own family. Yes, your husband has an illness, and yes there will be some behavior beyond his immediate control. However, that is not a free pass to wreak havoc on your family.
It sounds as if he is aware of his issues and is attempting to fix/manage those issues. Kudos to him for the self awareness and the fortitude to seek help from professionals. Now he just needs to accept responsibility for those issues. They are his to own and rectify. He needs to know his behavior matters and "all is NOT easily forgiven" when he has an episode. Again, it's an illness, I get it, however it is a manageable illness. Regardless of illness he is responsible for his own actions. You should not have to police yourself or your children for fear of setting him off. That's a horrible way to live. Not to mention it is just enabling him to continue having meltdowns because in his mind he knows there are no consequences for it. He can say and do whatever he feels like in the moment and then just use the excuse of his illness to sweep it under the rug. Knowing full well that you and the kids will alter yourselves to fit his needs.
Now you are worrying yourself sick over this vacation because you are afraid of what he might say/do in front of your parents. That is just way to much stress for one person to take if you ask me. It seems you never get to have the down time without worrying about his issues. Honestly dear, you sound like you could just use a break from it all :(
If it were me I would have a conversation with him prior to the trip. Simply state that while you are aware of his issues and support his treatment plans you will not tolerate him having a meltdown in front of your parents.
Remind him that they pray before meals, dad leaves the seat up, Mom talks constantly, etc. Anything that might trigger a negative response from him should be brought to his attention prior to the trip. Remind him that you are already on guard morning, noon and night, at home and are really in need of a RELAXING vacation for you and the kids. If he can not GUARANTEE he is on board with this and will not go off on a tangent on the vacation then he really needs to bow out now and allow you and the children the well deserved break from his reality.
Are your parents aware of his issues or would this all come as quite a shock to them? I would have a conversation with them as well reminding them about your husband's current mental state. Discuss his depression and anxiety issues with them. Include the details of known triggers for him. Of course no one is asking them not to pray before their meals however I'm sure there are other little quirks that may also be triggers for your husband that they could avoid. Meaning if politics set him off ask them NOT to discuss political matters, etc.
Just so you know, we are catholic and pray before all meals as well as bedtime prayers with our girls. However, when we have guests over for dinner we do not insist everyone pray with us. Those that choose to are welcome and those that don't that's no big deal to us either. They just respectfully wait for us to finish and then move on into dinner conversations. Now if we knew ahead of time that a guest (especially a family member) was struggling as your husband seems to be then we would have no problem discreetly saying our blessing as a family in the kitchen prior to calling everyone else to the table for dinner.
Peace and Blessings,
T. B