If you are staying in St. Martin, you won't want to go up valley to Val Thorens for dinner. The restaurants in St. Martin are much cozier, friendlier and the food can be seriously good. Again, remember: Lachenal! You'll be having lunch in Val Thorens where the restaurants get better as you get higher up the hills.
Think lunch if you want to eat away from your base, and be prepared, once you orient yourself to the area's style, to reserve for lunch. This isn't Killington chili! Try Arc en Soleil at the base of the Saulire cable car in Courchevel 1850. Ask for the Savoyard specialties.
The bus vs. train
This guy is crazy for not taking the bus advice. It is cheaper for a direct bus and faster. Fewer chances for a missed train connection too.
Grenoble airport to Grenoble city is a ticket and 45 min. ride. Train Grenoble to Moutiers is another ticket and 2.5-3 hours. Buying train tickets can take time. The bus is a direct is direct in 2.5 hrs leaving from the airport.
Train/bus station is pretty small. I don't think there are even seats in it. Not sure camping would be too smart.