Safely possible? Back feed generator through existing 3 wire OR convert to 50A RV service
Nope, this isn't going to work.
The lack of a seaparate neutral/ground is a deal-killer here on both applications.
Further, that 8/2 cable is not allowable to 50A.
Neutral is not ground
There's really nothing confusing about it, except that you bit fruit from the tree of knowledge when you noticed neutrals and grounds go to the same place in the main panel.  Disregard this: that isn't what you think.
Electric current flows in loops - you kinda know that.  It wants to return to source (not ground by the way; source).
Neutral is the ordinary, everyday current return path, and it has current flowing on it all the time.  Neutral &quot;works for a living&quot;. Because of this it can be hot under certain conditions, and that's why we insulate it.  Using a bare neutral would be dangerous.
However, trouble can happen. Equipment safety ground is a protective shield.   Current NEVER flows on safety ground except during a fault condition.  Safety ground returns fault current to source:  So for natural electricity (lightning, ESD) that is the earth.  For human-generated electricity, that is back to the neutral and the supply transformer.  (and now, you know what that neutral-ground connection in the panel is for).  If it wasn't for safety ground, the equipment would sit there energized.
Your pool heat pump ran on 240V only, and did not need neutral (which is in the middle).  The original installer saved a tiny bit of money by using /2 cable instead of /3 cable, which unfortunately ruined your ability to reuse it for your generator.
Your options
Since you need to pull new cable anyway, you might as well pull both of them - a 6/3 for the RV, and at least a 10/3 for the generator.  (A 12/3 would suffice for now, but there's a lesson here and I'm assuming you've learned it :)
The existing 8/2 could be used for a 120V feed to a generator, hot-neutral-ground, of up to 40A.  However, if your house is wired in &quot;Multi-Wire Branch Circuits&quot;, that could introduce a danger.
The 8/2 could also be used for a 120V/30A RV connection called a &quot;TT30&quot; found on smaller RVs.   If your larger RV has the adapters to work on a TT30 connection, that is an option.