My experience with bilge pumps has been that pumping 
feed is going to get you anywhere between half and 
two-thirds the gallons per minute claimed for the 
pump, even with a short hose and a 250-gallon tote 
on a trailer, making for a downward flow of feed
and good static head pressure.

In the hull of a boat, where they have no excuse
for it, I find that all brands tend to overstate 
their capacity by about 15% or so, but in a boat, 
the pumps are always below the water line, pumping 
water up.  

Clearly, the 100-foot hose is going to be the major 
source of additional friction in your set-up. 
I never tried a hose that long.  I'm impressed that
you were able to pump feed through a hose that long 
with a mere bilge pump.

If your hose is corrugated, rather than smooth, 
that's even more friction.  Opening up the fill 
port on the tank might also improve flow, which 
would mean that the vents are too small for your
thick syrup.  Everything is "designed" for thin
liquids like plain water.

An inverter on the truck to provide 110 volts AC
might expand your pump choices, but the other
pump brands that marine supply companies tend 
to carry with which I am familiar are:

Rule:
I'd stay away, based on their cheap float switches
 
Jabsco:
Never heard anyone cursing at one
         
Attwood:
These will be quick to melt if you let your voltage 
drop below the required 12 volts, but any pump will
die quickly if you run it for long at low voltage.  
(You can guess from this comment which pump I 
melted in my boat!)

I don't have a better idea on the air purging.

*******************************************************
* Search the BEE-L archives at:                       *
* http://listserv.albany.edu:8080/cgi-bin/wa?S1=bee-l *
*******************************************************