Orkimedezz has a good point there at the end. AV14 is hard to combat in a 1000 point list, unless you're playing against
IG or Eldar. Most other armies can't afford to take the big guns that will power right through it. I agree with the recommendation for leaving at least one big shoota on the wagon(s), because the deff rolla does not count as a weapon, so on a pen roll of a four without a weapon, you're immobilized. On the plus side, though, you can't lose the deff rolla on a weapon destroyed

. 5 points for a chance to keep your rolla smashing is a good investment to me, especially because you might actually kill something with it!
To reitterate, split the lootas in to two groups. The only reason to bring only one group is if you only have one slot left.
Orkimedezz also has another point, though he did not voice it directly, about the other wagon. Your MANS will get beat to hell if they're the death star of your list. Unfortunately, Orks don't really have death stars like marines do. It'll be tough to take out the wagon, but plasmas are cheap, and
AP 2 is the bane of a MANS' existence. Taking another wagon will increase your chances to keep theirs alive, as well as having a backup transport in the case the one gets taken down.
I personally don't trust trukks because they're First Blood fodder, but if you can manage to protect them, they will certainly get your boys up the board fast.
The "MANS in a wagon go krumpin RIGHT NOW" strategy is great on paper, but in reality it doesn't work. I've tried it more than once and they always just end up dying before they can do anything effective and before the boys can get into the fray.