I'm told Hobbs NM is booming (oil fields also). That whole basin is booming (Permian Oil Basin). Gets hot during the summer but winters are fairly mild. Not much snow, mostly overnight freezing temps (its a desert out here). NM has state income taxes. TX does not. Prices are pretty decent. Checkout the campground on http://www.campgroundreviews.com
for an idea as to site costs. Bear in mind you will be seeing mosty nightly costs so you will need to look for the monthly rates on the campground websites. Or you will have to e-mail or call for monthly rates.
This is not a hard and fast rule but I have found that you can often multiply the nightly cost by 12 or 13 and come real close to monthly rates. It's good for a general idea. Don't forget that RV's can often stay in mobile home parks (you may have to get your own electric and cable)