I think on current form Campbell is not far off any of DHP, Maddocks or Banks.
DHP is 31 and has again injured his knee -- rehabbing at 31 isn't the same as it was at 21, even then it's not like he's a 50 cap vet, he's only got 30 odd caps, and not in particularly good form. Banks is probably the most consistent of the lot, he brings some unique skills to the table for example his excellent kicking for touch (although sometimes puts too much pressure on himself and bites off more than he can chew). At his best he's probably not as Maddocks, but at his worst he's vastly superior to Maddocks. Maddocks obviously has had a few excellent games but also a few shockers too. Instinctively I feel like Rennie will prefer both DHP and Banks to Maddocks just due to the consistency. On the topic of experience nether Banks nor Maddocks have extensive experience at Test level, but they have a few seasons of Super under there belt, not sure how much that counts for once you are in the test squad; but I'd expect much of the squad to get a run for various games against the likes of Argentina, so anyone in the selection zone could really stake a claim.
If Banks can perform against the Reds then he could sure up his claim to the starting jersey but equally a poor showing could have him going backwards against the other two. Obviously if DHP is out for an extended period Campbell is the likely option to come into the squad, and I think if in the squad is even money to get a crack at the 15 jersey.