Ack! We plan to use our van in the exact same conditions with the exact same temperature swings. Would you mind elaborating? This is the first time that I've heard someone with firsthand experience mention these issues. Would it be better to not seal the bamboo at all? Where did you source your bamboo?
I'm in South Lake Tahoe, CA and had to drive down to Stockton, CA where I picked up a single sheet of 3/4" and four sheets of 1/2" bamboo plywood. 5 hours round trip but around here, at that time, if you wanted bamboo plywood that's where you needed to get it lol.
The drying out and bowing was only an issue with the small cabinet ends that I made. The larger wheel well boxes pictured are also 1/2" bamboo ply and had no issues expanding or bowing at all even though both examples were mounted to 80/20. I would bet that a large piece of 1/2" bamboo (i.e. cabinet door) that's basically free-standing other than the hinges that hold it to the frame, would warp very badly with temperature swings.
The sealant that I used was something off the shelf at our local store which I try and avoid but I thought I could get away with something less-specific than what I actually needed. I'm sorry that I can't recommend a sealant and I can't remember what lousy substitute I used. That van saw a lot of moisture so we sealed every piece of wood in there.
The 3/4" bamboo ply didn't/hasn't warped at all in my shop when laid flat. The 1/2" sheet of bamboo ply in my shop laying flat bowed up with temperature swings.
I think 1/2" bamboo s great when it's secured to another material (80/20, aluminum angle, etc.) but when it's left alone it can be more sensitive than a Baltic birch of the same thickness.
My main use for the 1/2" bamboo plywood was for our three panel aluminum bed. In that application, each piece of plywood was secured to the aluminum bed frame panels in about 10 locations per bed panel. No issues with bowing, no issues with the bamboo drying out, warping, etc.
I'll be using 1/2" bamboo plywood attached to 1" square aluminum tubing in our AWD Transit van when it arrives this winter as part of a custom sleeper-sofa design that I've had in mind for quite some time.
I'm sorry that this is probably leaving you with more questions than answers!
Here's a better look at the 1/2" bamboo attached to 1.5" aluminum for our three panel bed in our old Sprinter.