You said it takes you about two weeks to go through a tank of gas, is that because the truck sits for two weeks?
Do you only drive it on the weekends?
Everyday?
If it is not getting upto proper temp everytime you drive it it can set off codes, once in awhile not reaching full temp is fine, but not everyday.
Example, my wife drives her van everyday but only 3 miles or less at a time, so once a month I take it onto the expressway and drive it for an hour at highway speeds.
No codes, nothing. The van runs and drives perfect.