OP, my two cents:
1) Golf cart batteries. Greatly more "deep cycle" than the ill-named "deep cycle" batteries.
2) Amp-hours, amp-hour, amp-hours.
3) More amp-hours.
Since you can get golf-cart batteries in 6V, buy two 200AH units and put them in series to get a 200AH, 12V battery. Then, if you want to increase capacity, repeat, and put the next set in parallel with the first.
Not only can you use your truck to power stuff, you can also use it to charge the batteries.
There is another option, one of the UPS makers (APC? Tripp-Lite?) make some charger/inverter combos to which you add your own batteries. They can be configured to charge only, invert only, or charge AND invert when the power is out (making them act like a UPS.) Use one of those to keep your batteries charged normally, and you can power your 120V stuff off of it as well when the power is out.