Well, I'm assuming you have an establishing shot from the outside that shows how this truck has a translucent cover. Probably any white cover will work. After that, for the shots inside the truck bed possibly a frame with full grid cloth will work, provided we don't see the frame --which would be most difficult. A possibly less expensive solution than grid cloth would be to drape the frame in white parachute materials.
An interesting challenge, so go for it!
Rick Wise
Cinematographer
San Francisco Bay Area
http://www.RickWiseDP.com