Car mpg

My van is a 2012. I need help because I think
I'm going crazy. I've been ill and I'm disabled
and memory issues/brain fog have recently
become an issue that we believe is related
to an upcoming diagnosis of Sjögren's
Syndrome, but I swear this is not a memory
issue. Wednesday of this week, I drove
home and just before I pulled up my
driveway I noticed I had 5/8 of a tank of gas.
I have an 18 gallon gas tank. That was
Wednesday. I did not drive the car again until
Friday, to drive down to the end of my
driveway and get my mail which is about
eighth of a mile. I noticed that I no longer
had any gas. Not only was my fuel light on
but my DTE indicator had gone past telling
me how many gallons I had left to the
LOFUEL indicator which tells me I better get
gas quickly. I told my husband something
that happened to my gas and he had not
been home and we live in a rural area. (He
was in Dallas at the time from Tuesday until
Thursday evening). Today I took my car out
to get gas and put 18.53 gallons of gas in
my car to fill it up. I didn't even know my car
could hold that much gas. I reset my trip
odometer every time I fill up my car but this
time I noticed that instead of having
between 18.2 –23 mpg gas economy I only
had 13.6 mpg. I've never been lower than 18
mpg. I believe this supports my theory that
somebody took gas out of my tank. My
husband believes this supports his theory
that I'm nuts. Can somebody tell me if I'm
wrong? In all seriousness, I really need to
know if I'm wrong because I'm beginning to
think I'm losing my mind. Please help me.