Premium tanks do get a credit bonus; it's typically 130% or more of what a similar non-premium tank would get (I arrived at this number by comparing premium tank income to non-premium tanks that have a 30% or more credit bonus weekend).
The main money makers are the tier 8 premiums; a good round in one of those can clear 100,000 credits (gross; typically about 60-80,000 net, depending on ammo and repair costs) on a premium account.
You can see a great chart here that shows, over the last 30 days, the net income (after accounting for repair, reammunition, and replacement of consumables used in the match) of all tanks in the game (as if they were played on a standard account -- the chart is adjusted to account for premium bonuses etc). Premium account holders will generally double this net income due to tank costs being fixed.
If you are buying a premium tank, it's usually a good idea to buy one that shares a nationality and type with tanks you are actively working on; for you, if you're working the American heavy line, I would recommend the T34. Its playstyle will be familiar, but more importantly you can swap the M6/T29/T32/M103/T110E5 crews into the T34 and use them in that tank without retraining them in order to increase the amount of crew experience they earn.
One warning: if you have not progressed beyond tier 6 in any tank, be aware that your experience may be frustrating at first, simply because you are coming up against much more capable tanks (and much more experienced players), so it may take you a couple of hundred games in the tank to start regularly having decent matches. It may be worthwhile to buy a tier 5 or 6 premium first, use that to earn credits, and then once you have reached tier 8 "naturally", buy a tier 8 premium if you need/want to.
Another good way to earn credits is to run tier 5-6 tanks (on a standard account; premium accounts can make money at higher tiers as well), especially ones that currently have an income bonus. For these tiers I would suggest the M4 Sherman (use the 105mm with HE ammunition), the PzIV (likewise), the KV-1S (whose damage potential outweighs its ammunition costs), the Hellcat, or any other tank at tier 5 or 6 whose playstyle matches your own.
Note that tank costs are usually dominated by ammunition costs, so a tank with cheap but effective ammunition will often make more money than one that does more outright damage but has more expensive ammo.
Finally, if you are aiming to maximize credit income, do not fire premium rounds unless it may be the difference between a win and a loss (wins get 150% bonuses on XP and credits). They are very useful for maximizing experience, but will destroy your net income unless used very sparingly.
Spotting damage is also very efficient (since it costs you no ammunition), so running scouts (if you're good at them) can be very effective at bringing in credits.
Edit 9/4/2014: As of 9/12/2014, premium vehicles will receive additional bonuses. All premium vehicles will receive 1.5x normal crew XP (making them even more valuable for crew training), as well as a tier-relative bonus to all XP earned in the tank (which will stack with the crew XP bonus). Lower tier premium tanks will earn a higher percentage bonus, although this will probably not be enough to make them better on a per-game basis than higher tier (and more expensive) premium tanks (simply because higher tier tanks earn more XP on average). Prior to this update, the Churchill III (premium tier 5 Russian heavy tank) was the only premium tank in the game with an XP bonus (of 1.35) -- it will keep this additional bonus after the update, so it will still be the best crew trainer at tier 5.