Building Sides

I've read a good amount of pieces on building sides, but with so much information making a decision isn't easy
So here's what I need, the sides need to be 5x10 at a minimum, I can get 5x5 birch and spline it together, which is OK but I wanted to do a glue up of 1/8" exterior, 1/2" internal (with routing for insulation and wiring), and then 1/8" hardwood lam on the inside (cherry in our case). So the 1/2" birch would be covered up, and the extra cost seems like a waste, I was considering just some good 1/2" ply in 4x8 sheets, maybe marine grade.
You can't get the 1/8" in anything but 4x8, and I'm really thinking a woody exterior would be awesome. So that leaves me with some questions...
Is there a good way to join 1/8" pieces to stretch them?, or should I just glue it to the internal ply?
Joining 4x8 pieces is not hard, but the seams would be an issue, I don't want a seam on the door, or have the exterior and interior plys on the same seam...
This is the first time I've confused myself with this
And before anyone says something about the insulation, we do VERY cold weather camping, so it really is needed...
Thanks
-Paul-

So here's what I need, the sides need to be 5x10 at a minimum, I can get 5x5 birch and spline it together, which is OK but I wanted to do a glue up of 1/8" exterior, 1/2" internal (with routing for insulation and wiring), and then 1/8" hardwood lam on the inside (cherry in our case). So the 1/2" birch would be covered up, and the extra cost seems like a waste, I was considering just some good 1/2" ply in 4x8 sheets, maybe marine grade.
You can't get the 1/8" in anything but 4x8, and I'm really thinking a woody exterior would be awesome. So that leaves me with some questions...
Is there a good way to join 1/8" pieces to stretch them?, or should I just glue it to the internal ply?
Joining 4x8 pieces is not hard, but the seams would be an issue, I don't want a seam on the door, or have the exterior and interior plys on the same seam...
This is the first time I've confused myself with this

And before anyone says something about the insulation, we do VERY cold weather camping, so it really is needed...
Thanks
-Paul-