Also I believe your math may be a bit off as to the amount of fuel a given engine will consume per hour.
The normal thermal efficiency of [biogas] engines is 25-30%, and they use approximately 0.45 m [i.e. 15 cu ft] of biogas per horsepower-hour.
I would be more than comfortable with enough gas to travel say 100 miles per day which... translates to what? about a 3000 or say 4000 gallon digester That's probably doable.