All this is speculation so I may as well jump in as well.
Looking at the location of the camper in the parking lot (away from the store), as well as the motor home next to it, it looks like the person was camping. It looks like it is early in the evening (by the parking lot lights being on and the fact that someone was at a Starbucks so more than likely at night - yah a guess). So could have been ignited by the furnace being turned on or cycling. Gas build up could be from a stove being slightly on or a leak around the furnace (the hot water tank and fridge is around there as well). Also seeing that the camper is split close to the right side, that would be where the furnace is located (at least in my NL that has the same layout). Could have been a leak in that side and the propane was trapped in the closet, build in area around it.
There that is my 2 cents. I would really like to know what happened.
On edit: I was wrong on the time as it happened at 5:30am.