is this right? fuel mileage in my truck sucks

[67cst]

hey guys, i put 70 bucks of propane in the truck exactly 1 week ago. i drove 280 miles or 450 km in the last 7 days. propane is 56 cents a litre or 2.11 a gallon. so i figure i go 280 miles divide by 33 (galons burnt) and i come up with just over 8 miles to the gallon..is this right? that is pretty poor mileage for a 350 on propane isnt it? maybe im an idiot and i did this all wrong, i sure hope so...if im right what kinda things can i do to improve mileage, btw i drive the truck like an old man, im not pinning it every chance i get

thanks Derek