As far as I am aware, there is no way to get more then a single attack off per turn with the heavy crossbow. Is this true or not?
The trick is reducing reloading to a free action. If you can do that, you can full attack with the heavy crossbow
You could use a repeating heavy crossbow which allows the wielder to make 5 attacks before reloading, but then reloading it is a full round action.
Alternatively, the 1st-level Sor/Wiz spell ghostly reload [trans] (Races of the Dragon 113) reloads the weapon for you but has limitations: it's 1 reload/caster level, maximum 10.
There is also the magic weapon special ability quick loading (Magic Item Compendium 41-2) (+1 bonus; 0 lbs.) that allows storing 100 bolts in a crossbow and reloading a heavy crossbow as a move action (or, if you want, a light crossbow as a free action). Talk to the DM about using the weapon special ability quick loading in conjunction with the feat Rapid Reload (Player's Handbook 99) to allow reloading as a free action so full attacks can be made with a heavy crossbow.
A self-loading crossbow (Arms and Equipment Guide 116) (12,350 gp; 8 lbs.) is a +1 heavy crossbow that can be reloaded by taking a move action. Combine this with the feat Rapid Reload so that the wielder can make iterative attacks. (Unlike quick loading, self-loading explicitly says the combination works.)
Snappie’s answer covers all the reasonable answers I’m aware of, but there is a cheesy combo that works RAW. That is an aptitude heavy crossbow with Hand Crossbow Focus (aptitude is a +1 special property from Tome of Battle, Hand Crossbow Focus is a feat from Drow of the Underdark). Aptitude allows a weapon to benefit from feats for other weapons, and Hand Crossbow Focus allows you to reload a hand crossbow as a free action while also working like Weapon Focus. So aptitude lets your heavier crossbow still work with Hand Crossbow Focus, which always provides free-action reloading.
This trick gets better, though, if you take advantage of another property of aptitude—it allows you to always use the weapon proficiently. That means there is no problem putting it on the great crossbow, which has much higher damage, and you don’t even need Exotic Weapon Proficiency.
Ultimately, I do not really recommend this. Aptitude is poorly written and easily the most broken thing in an otherwise excellent book. They probably only meant switching feats that give you a choice to work with the weapon, similar to the warblade class feature weapon aptitude (which would be kind of weak for a +1 but not hideously so when combined with the auto-proficiency). But that isn’t at all what they wrote and so now we have this broken mess.
Although it's an epic feat, with all the problems that entails, Instant Reload does allow this. I'm unaware of any other methods to achieve the same result.