I personally think you're ok doing either of these.

A: Installing a second battery in the back, and no new alternator.
B: Installing the Power Inverter in the back with no alternator or battery.

I've ran multiple batteries, and 3,000 watts of amplifiers on a stock GM alternator, and it never went on me. I'd say start with Option B running a 0 gauge wire to the back of the VX to a fuse then to the Power inverter. Put it under the load you think will be typical while you're camping. If all seems well, roll with it for a while, keeping an eye on things. If you need to add a second battery, you'll already have the 0 gauge ran back there so installation will be a snap.

Thinks to keep in mind, Make sure you ground the battery with the same gauge wire as you ran the power. IMO Battery Isolators are unnecessary, but you may want to put one in if you're worried about the load on your alternator.

Worst case scenario, you start with just the inverter... then add the extra battery... then end up adding an alternator down the road. No need to get hit with everything at once.

Just my 2cents