Have a discussion with Neelco.
Failing that, I suggest, add a rubber extension to the pickup tube. Use a short section of radiator hose. It could be difficult to work it onto the lower end of the tube--so--unscrew the tube from the top if you can. Clamp rubber part securely with a stainless steel screw clamp. Keep in mind that transport and the associated sloshing can introduce air into the tube and this may result in an air break (air lock or cavitation)--perhaps you could increase the suction and suck out the air, by routing all flow back into the tank through the agitation temporarily. Diaphragm pumps are not designed to suck air. Park on the best slant you can, of course. Perhaps unbolt the tank and reinstall with a slant to aid full pickup. Be prepared to suck up whatever residue lies on the bottom of the tank--fiberglass, right? You could always ask Neelco to modify the tank so that the suction comes out from the bottom of the tank.
Let us know if Neelco helped you.