2016 crv Lx AWD here, only had it a week so far, 26k miles, I was getting down to 26mpg when it was 12deg F out, cold weather reduces mpg alot.
Yesterday I got new tires and the alignment done, it was well out of alignment probably from driving over the bad potholed Michigan and Ohio roads. My 350mile drive back since the alignment and warmer 35deg F weather was 33.7mpg.
Outside temperatures seem to affect alot of vehicles mpg, in my 3rd gen Prius cold temps significantly affected mpg & driving through rain did too to a lesser extent.
Still 18mpg seems considerably worse than average, even at cold temperatures. Might want to take your car for a check up, get them to check that your brakes are releasing properly and that nothing is rubbing and alignment is ok.