Well, you can always get a pure sine wave inverter, which -
Other then getting a pure sine wave inverter, what other things can I try?
Ah. So you understand the problem and why it's happening, and are hoping for an easier/cheaper solution.
One option is to get a laptop power supply that uses the vehicle power. These are relatively inexpensive, and may not only resolve the problem, but be more efficient overall.
If it's a low power requirement and a high power AC inverter, you could use a high power isolation transformer between the inverter and the laptop power supply. This will act as a huge filter and force the signal to be more of a sine wave, cutting out a lot of the higher frequencies. Could also damage the inverter, and this isolation transformer will heat up quite a bit as it's being abused. Follow that with a power line filter and you might resolve the problem, though at a significant cost in money, weight, and efficiency.
Beyond that, though, there aren't any magic tricks you can use to get rid of the hum without further investigation. You may be able to add ferrites along all the power lines and signal paths, isolators to avoid ground noise, etc, but the signal is very strong and won't be easily removed.
Try to characterize the noise - it is a single frequency, is it broadband, what is the signal level? If it's a typical cheap modified sinewave inverter, you're going to have a lot of high power harmonics, it won't be a simple fix or filter.
Once you have that information, ask for the specific methods to filter that type of noise.
My expectation, though, is that you'll find it's cheaper and easier to simply use the right power supply in the first place, than to use the existing equipment and stack filters, isolators, etc on top of it.