I think one of the New England Firearms handi-rifles would make a Jim Dandy cheap truck gun. For varmint/pest control, the .223 should be good enough. Lots of folks think the Mini 14 makes a good truck gun, but I honestly do not think it accurate enough, and it sure aint cheap anymore. Any of the AR type guns would be great, but once again, they are not cheap. Just my opinion.
As was the case for many, many years a good lever rifle in the caliber of your choice. Winchester, Marlin, Rossie, Mossberg, and others work well in 30-30, 32, 35 or some of the older calibers that tamed the west. You can pick one up used for a very reasonable price.
Another good choice might be a Savage bolt action rifle, again in the caliber of your choice. For the huge sum of $300 you can get something that do the job and not make you sick to your stomach if someone walks off with it.
Welcome fellow Texas country boy. Maybe I am old fashioned, but the Marlin or Winchester lever action .30.30 is the perfect ranch rifle. Big enough round to take most things down in Texas and it is very durable. I have kept a Marlin in the truck for years now...gets bounced around, never cleaned and always works just fine.
Depending on where you're at...you may want to consider whether a shotgun best suits your needs. If that is the case, I recommend
either a double barrel coachgun with interior hammers or a remington 870. I personally know that both will hold up fine in that environment as well. Good luck. -SBH