If you are going to being putting a 30 gallon tank in and only plan a few trips where you may need a little extra how about a gas can or two secured in the bed in case you need a few extra gallons. 30+ gallons should be fine for normal driving.
naw i hate that. the places i go and the way i drive everything bumps all over the place and the can always leak all over the spare, jack, chain box, coolers, boots etc. too messy and pointless for me