Totally not vehicle related, but ideas to reduce your overall size and/or weight...
-Go cabless! Especially if you have a PA system. Pedals like Sansamp and especially Amplifirebox pedals have all come a long ways in terms of sound quality and features. We run these and I (as a bass player) have not turned back. I have my pedal board and my basses, thats' it. Cab volume and weight be damned! My guitarist has the amplifirebox pedal and has since not used his 4x12 cab. Setup is much easier, transport is easier, and my back is happier.
-For your PA system, look into swapping drivers to neodymium and your amps to class-D/T. Those can shave a LOT of weight! Can also look into DIY cabs with quality ply to reduce weight.
OK, this probably won't change your vehicle perspective, but these are a few things I've done myself to make life a little easier for gigs.