I think you're right about going back home to get money in some way.

I know they were camping in that van up until that fight, but suddenly they were staying in hotels and eating at restaurants once he came back from the trip home. Recall that the final day she was seen alive, Brian had a big fight with a restaurant in Jackson Hole, was aggressively yelling at workers there, and Gabby was in the background crying.

Also after the presumed murder, someone reported that he offered them $200 cash to drive him a relatively short distance.

So it does seem like he got some kind of cash infusion after that trip home.