Time to get back to basics...
Do you game in the vehicle?
Do you game longer than the battery can last?
If yes, get a bigger inverter. You can wire it in just like an amplifier and hide it under the rear seats.
If no, turn the laptop off to charge (less current draw). The factory inverter may be adequate (try it).
Do you need a computer with multi-tasking capability to use while traveling?
If yes, how about a netbook? More processing power than a tablet with power requirements that fit the factory inverter. Used netbooks are cheap (everyone wants tablets).
If no, try a tablet. Great for multimedia or communication on the road. Decent tablets are available new for under $200. And, the power requirements definitely fit the factory inverter.
Gaming laptops are great fun and also powerful enough to replace a powerful desktop for road-warriors. They just aren't practical for use in the vehicle without special preparations. I leave mine in the case and use the netbook for most computer-related/non-gaming tasks while traveling. I use the tablet (or phone) for quick communication and multimedia entertainment. I pull out the laptop for presentations and demos (and gaming in the hotel room).
As always, YMMV. Check out NBR
for laptop specifics (I assume you know NBR, having a Sager).