Two 40s wouldn't fit in the doors to the propane bottle shelves. The 30s barely fit. I've never had to get up to swap out a tank or switch over. Checking them is just something I do to avoid that squawking sound from the right side of the bed at 3am. The thought of three 20s is more for the convenience of hitting the nearest Home Depot or even 7-11 for an exchange in places that don't refill propane bottles. That and they're lighter and more manageable. The 20 lb bottle for the grill rides in a milk crate in the truck bed now so I always have a spare just in case. The remote monitor is interesting too. If my regulator goes bad, I'll look into one.
And thank you for your service!