1) Insurance scheme...here is why:
a) Broken Furnace
b) Everyone is away from the house...hotel to Casino. Friend's house.
c) Cat is boarding. Seems odd? I could explain it, but still the cat was gone. Slightly off
d) Mystery van could be assistance.
2) Attempted Hit
a) Ex-Wife and Boyfriend in the house still owned by husband (or co-owned).
b) Mystery Van could be husband/"hitter" (yes I know that a contract killer is a weird one...but they exist and they aren't all "pros" like you see on TV).
c) A natural gas leak is a good "accident"
d) Broken furnace could have been the first attempt and it didn't work. Van tried to make it happen faster.
3) Something Bigger Picture
a) Drug lab? Unlikely but possible if there was a gas leak as well? (Occum's razor doesn't really fit that one).
b) Wittness Protection program? Somebody got found out? White van and some "quiet nice people" getting killed. Good timing with the other family gone. Heck could the "furnace problems" have been a way to get those people out of their home?
I am not investigator and I am sure I come off as a conspiracy theorist, but these all seem like possibilities here.