You've got some writing habits that are obstructing the story a bit. Sorry, but there are a few. Don't feel discouraged though, they're easy to fix.
First up, is this story meant to be in the present tense or past tense? You mix and match the two.
Compare these:
"Diane took pulled down her scarf from her face and pulled her companion close."
I'm assuming that "took" or "pulled down" is not meant to be there. Ignoring the "took," this sentence is in the past tense.
"Diane and Georgia without a word grab their shotgun and open the door into the darkness to see what caused the other vehicle to stop."
This sentence, however, is in the present tense.
There doesn't seem to be anything to cause a tense change, so I'll assume this was a mistake. I'd advise going for past-tense, rather than present-tense, because present tense is harder to remain consistent in.
Secondly, your sentences are a bit too long. Try to keep only one idea in each one. For instance:
"Rain, that was all it seemed the world was now, it came in waves, sweeping across them over and over again eternally pelting the front of Diane’s windscreen, she could feel the tires and makeshift tracks of her campervan slipping on the impossibly deep mud."
This could be instead:
"Rain
. That was all it seemed the world was now
. It came in waves, sweeping across them
. Over and over
, it was eternally pelting the front of Diane's windscreen
. She could feel the tires and makeshift tracks of her campervan slipping on the impossibly deep mud."
Long sentences are commonly used to slow the pace down. The pace is probably meant to be slow in this beginning part. However, short sentences -- while quick -- help increase the tension. Sentences that have more than three commas generally need to be edited. =D
Item three, whenever there is dialogue in a piece of fiction, the general rule is to begin a new line. However, that is not quiet necessary. So long as you have one piece of dialogue in each paragraph, you'll be good to go.
This is because paragraphs are supposed to contain one main idea each (much like in an essay), so when a new person begins speaking there is usually a new idea being begun. ("being begun," now that's an odd phrase).
Building off of that, your paragraphs could be split up a bit. "Walls of text," are scary. Small paragraphs are easier to grasp. That's why mine are always a few sentences long! Nice and bite sized! (But you should probably not go as small as mine. I need to work on making mine larger). I'm guessing you're used to essay writing?
There are exceptions. However as with most exceptions in writing, one needs to understand the effects that will be caused by breaking the "rule." For instance, dialogue mid-paragraph is reserved for quotations, and doesn't often show up in fiction. In fact, I can't think of any occasion where dialogue mid-paragraph ever is used in a "correct" manner. I'm sure some exist. Maybe in a stream of conscious piece of writing.
Example time:
That’s how they had gotten here, stuck in the mud, surrounded by hundreds of people but feeling completely alone; the radio had been silent for the week they had been on the road… their once pleasantly chubby figures worn down to skeletal skin and bones.
As they got back into the van Georgia laid the gun down on the table and immediately took up the microphone and screamed: “Is anyone fucking out there!?” into it, before falling to her knees utterly defeated. Diane came in behind her and shut the door, in the awkward silence of the rain hitting the roof and both their breathing a faint voice was heard for a fraction of a second on one of the channels.
They both sprung into life, quickly tuning the archaic devices trying to hear it again eventually after great effort they hear: “This is the Research Ship RV Odyssey broadcasting. Any survivors please state your GPS co-ordinate location on this frequency for flyer pickup. Message will repeat.” Hearing the message only once Diane immediately pressed the transmit button and read aloud the co-ordinates their infrequent GPS locators had them as being at, before checking those numbers on one of the old paper maps she had stowed on-board.
If I was writing this, I would have so many more paragraphs. But that would be bad, because mine are too tiny. So I've just put them where dialogue needed it. =D
And I think that's enough to be getting on with. =D
I like how you began the story in media res. Plus, the "how we got here" portion was good. It was a nice recap, giving information efficiently in order to get to the meat of the story. Although I'd consider there to be a bit too much tell, this is your piece of writing, not mine. (Plus, recap kinda needs to be tell rather than show). Overall, I'm excited to what will happen next!