What follows is a fairly usual tale of out-of-towners up to their necks in the Texan outback where seemingly everybody is in league with the cult (including the Sheriff) and is out to get them although they mask their evil intentions behind that brand of polite local friendliness seen in stuff like The Texas Chainsaw Massacre. The real selling point of this movie is the climactic car chase where various locals pit their trucks against the RV causing more than a little destruction along the way.
It's pure Grindhouse trash of course, but great fun all the same. The Satanic cult is extremely generic with as much thought put into it as one might expect from a low budget movie made in the 70s. People seemed terrified that this kind of stuff was going on all over the US at that time but I've never actually heard of any evidence of real sacrificial cults. It was just one of those paranoid knee-jerk reactions we've seen before in the witch hunts of the seventeenth century and the mass-panic over communism in the 50s.