Does mil give each person all of their gifts at once, or hand them out one by one? Not that it really matters other than if it is one by one, it will be harder for you to take your gifts and leave. If it is all at once, you can stay the amount of time you and dh agree on, then whip out garbage bags and pack up your other gifts and have dh start carting them to the car while you collect kids and their gifts.
Two suv loads? My, oh my. I am curious as to what gifts you give each other that would be so big or do the big ones come from mil? Either way, WOW! On a side not, where are all these gifts kept at mil's house--it would have to be in another room!
I agree with DPumpkin that you have options:
1. Open yours all at once "Hey guys, we'll be opening up all of ours at once here, just to let you know." This could be hard if mil parcels out the gifts one by one or throws a fit if you do so..
2. Open them at home and write TY notes/calls for them. You would have to get them from mil and who knows how hard that would be. Also would she let you take them and why would you want to sit through everyone else opening theirs?.
3. Don't go to the gift exchange at all - pick up the presents another time. You could open up just the ones from her while you are there.
4. Go for a set number of hours so that you hit some gift opening and a break for chatting, then leave before the next round starts. You will get to open some gifts but not all and will probably have to get the unopened ones later.
If you think that she will not take a break for a long time to keep you there, set your time limit and when it comes up, say your goodbyes and go. Ignore their protests with smiles.
Also, I would start shopping now--use coupons and sales and find ONE nice item per person for their gift--or a collection of items that have a theme and fit in one box or bag. They don't need to know how much you paid. At least you can do that as your part to cut down on the enormous volume of gifts.