Most of my lists are footslogging Orks but I have played a couple games with mechanized. My lists were nearly identical to yours in terms of vehicle choice: 2 x Battlewagons with Deff Rollas and 4 x Trukks.
I recommend giving all of your vehicles red paint job.
The Wagons are resilient with 14 front armor but if you are flanked you might be in trouble since they become armor 12/10 so be careful of that. One way to mitigate this problem is to put a Big Mek in one of your vehicles. He'll grant 4 up cover to his vehicle and any vehicle within 6 inches. The drawback to the Mek is that they aren't good for much else and now one of your Nob units don't score since you only have 1 warboss.
A list like presents an opportunity to take Ghazghull Thraka. His slow and purposeful drawback isn't as relevant with a vehicle. He also lets your Nobs score as troops just like a Warboss does. If you can find the points consider using him.
If possible mix up your Nob weaponry and add more variety. Bosspole, Waagh Banner and Combi Weapons are all useful and they allow you to split wound allocation strategically in your favor. With a 10 man unit I'd do the following: Painboy, Power Klaw, Big Choppa, Combi Scorcha, Combi Rockit,
TL Shoota, Bosspole, Waagh Banner, 'Eavy Armor or Cybork Body, No Upgrade. By doing that you now have 10 completely different Nobs for wound allocation.
Give your 12 man boyz units Boss-poles.
Nice list! This strat is really solid and it's a really fun way to maximize use of Black Reach Orks. Good luck building those Battle-wagons!