For a thing to exist there simply has to be a thing to exist.
Introducing absence here, as with the introduction of nothingness with relative nothingness, only serves to complicate and confuse the matter.
In essence you are claiming to determine what is by what is not. However such does not follow. What is is determined by what is. What is not is not, thus is not determining.
You are attempting to explain being through non-being, through absence of being. That does not clarify understanding, it convolutes understanding. Being is explained by what is, not by what is not.
A tree is a tree because it has characteristics of a tree. A car is a car because it has characteristics of a car. Qualities, things are referenced to contrast and compare other qualities and other things. Things are relative.
Relative nothingness is a misnomer. Relative nothingness does not involve that which does not exist but rather that which is located or identified elsewhere. What relative nothingness actually implies is relative absence or contextual absence, terms of which accurately convey the idea.
Relative nothingness is an awkward, forced concept. Nothing is forced into the equation although only things are involved. At no point is nothing or nothingness itself actually present or actually identified. Things are identified, things are compared and contrasted with other things, not with nothing. Presence is involved, absence is involved yet those are not the same as nothingness. The term nothingness is needlessly and inappropriately attached to an already sufficient concept of relativity.
How does one even use such a term? “There is relative nothingness”? “There is a relative nothingness of funds”? “There is a relative absence of funds” is much more practical.