I'm not a newbie but I'm just getting back into flying after almost 6 years! I've got a sig somethin extra arf that has been flown many times, and was well balanced. I recently put a new .46 thunder tiger on it ( I LOVE IT).
However in level flight it tips heavy to the LEFT, so I have my trim set 80-90% to the right to compensate.
After getting home, I tested the lateral balance by hanging from some string, and it actually is quite heavy on the RIGHT side.
Maybe the muffler is heavy?
But how is it that it's right side heavy, yet I need so much right trim?

Could be that as it sat for so long it developed a warp. Eyeball the wing or put an incidence meter on the tips and see if it's not twisted

Well I don't have an incidence meter, and just did a little surfing to find out what that was. I eyeballed the various lines on the plane and saw no noticeable skews or warps. I'm sure nanometers count on stuff like this though.
So my next question is, if there is a difference in the readout of said incidence meter, how is it corrected?
I'm guessing the monocote comes off and some torque pressure is involved, or sanding?
Also, should I even make an attempt to add weight to the left side? I don't know if on her next flight I'll have enough trim left to level her out if I add weight to the left side.

Could be a possible warp in the elevator!

Right side heaviness calls for lateral balance adding weight to the tip of the left wing; that will solve static inbalance.

The trim to the right means that the plane has a dynamic inbalance, which means that the right side surfaces (wing-aileron+stab-elevator) are lifting more than the left side.
When the areas are identical, the surfaces with higher incidence respect to the airstream (AOA) lift more and induce a roll.
It does not take much angular difference to disrupt the balance or symmetry the lift.

To eliminate warps, twist the surface in the opposite direction while you hit it with a heat gun.

Measuring the LE's and TE's from a flat surface can be almost as good as using an incidence meter.

Yep, my left side elevator is higher than the right side.

And it rolls left, right.

Yup, you figured it out. Now do what it takes to make them line up.

Hi!
If your left elevator half is more up than the other then the plane should turn to the left...!
Most of the time that a plane turns drasticaly to one side it's the wing that is twisted!
You better check it out using a robart meter. But eyeballing works to! Just look from the wingtip in towards the wing root...and compare the angle (The bottom angle of the wing)