Jeeze-louize! We are definitely mapping out some seriously crazy terroritory . . . I like it!!
Okay, new idea (while I go off and have a bit of a think about this). In the simpliest of terms, since you started kinda close to there to begin with, and since the ideas were kinda converging anyway, why not simply have a matched set? Iow, have what might be a megahaunt in two parts. The first haunt is the plantation idea, or, rather plantation style house . . . doesn't need to be real plantation-y, if you will, but can be more along the lines of . . . . eh, wait . . .
OKAY, GOT IT!!!!!! SWEET!!!
Alright, two haunts: a) The Family Home; & b) The Truck-Stop out front, which heads off into the swamp campground, a la Greg's brill suggestion. Greg's "ancient evil"? Cthulhu, the Ancient One!
The two themes: a) Welcome To The Family (Psycho, House of 1000 Corpses, TTCM); & b) Ancient Evil Awoken (From Dusk Til Dawn, Prince of Darkness, In the Mouth of Madness, Exorcist, Omen).
The two settings: a) Plantation Style Manor (ref: '03 Texas Chainsaw Massacre) fashioned into a "cozy" B'n'B, with flesh and bone furniture; & b) Truck-Stop fronting Greg's crazy swamp horrors.
The two motifs: a) '03 "Texas Chainsaw Massacre" meets Arkham Asylum & Miskatonic U; & b) '09 "Friday the 13th" meets "From Dusk Til Dawn" meets "Gator Bait" meets Lovecraft's Cthulhu Mythos.
The two sets of monsters: a) "The Family" - cult high priests/esses and usual suspects; & b) ". . . and Their Minions" - worshipers of their Cthulhu-Cult, who do their bidding, plus various swamp monsters.
The parents could be prepping to be sacrificed when they are interrupted by the hitchhikers or broken down travellers (the patrons). "Oh, why, do drop in!" The Ancient Evil is alluded to in the first haunt and is perhaps only a figment of their imaginations, or derrangement, or delusions. Watch the '03 "Texas Chainsaw Massacre" for reference, and re-imagine them as stereotypical "bible-thumpin' fundamentalists", only they are "necronomicon-thumpin' cthulhuists", if you will (ref: last scenes of "Rosemary's Baby"), and gradually you can salt the sets with torn out pages from the necronomicon, and then start smearing cthulhu incantation glyphs on the walls in blood, and so on.
You can even sneak a little black-light or UV into the mix by having a room lit only with a TV left on, to a blank station with white static, illuminating a wall of blood smeared glyphs, and then when the patrons get halfway thru the room, a UV strobe hidden in the TV starts strobing, revealing hidden writing in UV sensitive paint that says "Y'all R Dead!" and "Hiz Got Yer Soul!" emblazoned all across the wall in various print sizes. That's a very cheap effect to achieve, and it would look wicked cool. You could even scrawl a thumb nail sketch of Cthulhu in amongst the UV captions, which would be rather unsettling. You might want to also offset this with a loud animal roar of some sort, like a cross between a pig squeal and Regan's demonic, possessed caterwaul from "The Exorcist", hitting them from behind from where ever their attention is directed. Whatever the sound is, just make sure it's loud and shrill, like running fingernails across a chalkboard.
In the second haunt, we discover that apparently that Ancient Evil is, in fact, real. Not only that, the campground behind the Truck-Stop is built over the resting place of Ancient Ones who are hibernating deep in the ground, waiting to be awoken so they can reek havoc on the Earth. If you want to add zombies to the mix, have them be proper voodoo zombies, under a spell to do their Masters' bidding, and showing signs of hosting the Ancient Evil; vs. simply being undead or infected by a virus (which, frankly, is way overdone, unless of course you are making a proper go of doing a Zombie Apocalypse). We have all of the evil minions doing their psychotic thrall thing, and that would adequate, but if budget permits, you could also have all sorts of various swamp creatures, like that hogzilla animatronic you were asking about, tho I wouldn't blow big bucks on that until you can determine if you can make it yourself for less.
If you can get more into swamp creatures along with the possessed minions, you can start with basic animals and then evolve into more demonic Lovecraftian creatures. You might also be able to work in some sort of Lovecraftian styled Were-Boar, for instance, which would be interesting. That would take the concept of Hogzilla to a whole new level. There is also the temptation to do a stilt-costumed Cthulhu, but unless you can pull that off convincingly, I wouldn't go there. He'd have to be at least 10' to be convincing, and he would have to look super cool. Some cheapo bat wings and a rubber octopus on someone's head would definitely not cut it.
To recap, in the first haunt, you have the plantation style home straight out of TTCM, the horror within superficially disguised as a B'n'B, where different rooms have the remains of past guests, and we gradually go from Psycho to Delusional as we start seeing a cult scenario start to unfold, and realize that this family is not only cannibalizing, but self-cannibalizing, not to mention self-messianic.
In the second haunt, we go thru the Family's truck-stop, and things don't seem right, building atmosphere. Nothing scary needs to happen. We just build anticipation, and set the stage. The TV in the diner is left on, which starts setting up the story. This could be queue line, or this could be the first room. I'd keep it first room, so you don't break up the cool atmosphere with a hall monitor reciting the rules. Either way, the hard core scares start coming when we make our way thru the kitchen, out the back door of the truck-stop and into a picnic area, where we get a closer look at the various food and fixin's, seeing evidence of cannibalism, and encounter our first victim, or perhaps our first monster. We walk thru that section, and follow a trail leading into a campground which was ill-advisedly built into a swamp, and over the resting place of some Lovecraft "Ancient Ones", and then gradually everything goes downhill from there, devolving into swamp-bait madness, per Greg's various ideas.
See, you now have all the proper elements to make a great haunt, and you haven't spent a single dime. That, my friend, is the $4 Fix.
More in a bit . . .
Btw, the Family's name? "The Geinys." The name of the Truck-Stop? "Mama Geiny's Happy Highway Hospitality Truck-Stop, Campgrounds, and Grill."