Is it possible to get grenades for an RPG? Is there any case law for using an RPG in self defense?
Even for inert training rounds without explosive, they're
very expensive and you'll need federal licenses that are hard to get and require local LEO approval etc.
There is not really any serious case law above lower courts for the use of any NFA items, whether suppressors, full auto rifles, or destructive devices in self-defence. There are two stories that are commonly cited in regards the use of full auto weapons in self-defence, Massad Ayoob did writeups on both that tell the general story.
One involved a Florida gun shop owner named Harry Beckwith who used a S&W M76 submachine gun (the .45ACP version of the Swedish K) and an AR-15 (semi) to run off thieves breaking into his gun shop. Beckwith had a good relationship with his local law enforcement, probably let the chief dump a few mags through the M76 after the two incidents he was involved in, he never got charged. You can't count on that.
The other involved a H&K salesman named Gary Fadden. Fadden got involved in a shootout with outlaw bikers that he claimed was unprovoked (no dashcams in 1984 so who knows...) but got charged because a) he was in VA b) he went full rock and roll with a Ruger AC-556, a fully automatic Ruger Mini 14, against guys with shitty pistols.
Fundamentally there's no reason to fuck with NFA shit as a self-defence strategy. Fully automatic weapons or bombs or rockets don't give you substantial advantages beyond semiautos that outweigh the risk of making yourself a target for an ambitious prosecutor.
On another note, as you mention swamps, I assume you're considering returning to the home of the Florida Man. If you are, you need to make sure to take care on another front- Florida has some of the worst anti-militia laws in the US. A bunch of guys from the American Front skinhead group got charged for 'paramilitary training' for basically just 'doing some sparring' and 'shooting into a a dirt mound' 'while racist' and a bunch of them got fucked over. Those guys, like the supposed 'Gretchen Whitmer kidnappers', never had any intention to do anything serious in the real world, but both groups still got railroaded. Given your profile, it might be sensible to avoid doing anything too 'paramilitary' with the boys.