Towards the end of 508 Gliding Over All. Walt and Jesse are awkwardly reminiscing about their times cooking in the RV.
At one point, Jesse brings up the point of how much money they had made at that point and wonders why they didn't buy a newer, more reliable vehicle.
Walt replies with one word:
Inertia in it's simplest is one of Newton's laws stating:
An object in rest tends to stay at rest, an object in motion tends to stay in motion.
What does Walt mean by implying inertia kept them from buying a new RV?