A power supply will only draw what is needed for the demand. Deliberately under-sizing it will not reduce demand. The machine will just pull more than the supply can handle and the supply will die, possibly dangerously.
Without access to wiring modifications the best thing you can do is shut down the laptop and let it charge. Unplug it before starting up. The limits you are experiencing are a warning. If you ignore it you could damage something.
I would look at the vehicle's fuse panel to see what size of fuse is used for the accessory socket. Note that it might be shared with other accessories like a stereo which would leave less surplus. Also you want to check the manual for the inverter. See if it is 400W continuous or peak. It might have a lower continuous output below what you need.
If you can confirm that your accessory plug is fused at 20A then perhaps a different inverter *may* be able to work but it depends on efficiencies. Inverters are not 100% efficient so even if you have a 15A socket you will not be able to invert to AC, convert back to DC with the power supply, and expect to still have 180W of usable DC power.
A bus station is where a bus stops. A train station is where a train stops. On my desk, I have a work station…
