As long as a tornado has uninterupted contact with its fuel (warm air, moisture) and the atmospheric dynamics that got it going in the first place (shearing, twisting winds, dry mid-level air, a fast-moving jetstream above it), it will continue to operate efficiently.
Here's more on how this tornado engine works. As a tornado moves across the landscape, the fast-rising air within the spinning vortex leaves behind a small area of low pressure at the surface. This makes sense, if you thnk about it. All that rising air being sucked up into the top of the storm by the tornado would have to leave a void at the surface (an area where there isn't much pressure) because most of the air molecules have been removed.
Of course, where's there's a void, there's usually something else around that tries to fill it. In this case, it's the surrounding air, which is now under higher pressure than the vacuum beneath the tornado and rushes-in to fill the gap. As long as the tornado vortex isn't disturbed, surface air will continue rushing toward the tornado's base and be sucked upward through the vortex, keeping the tornado engine running efficiently.