The sum of all coefficients in the expanded form of $(3x+1)^n$ can be obtained by setting $x=1$. This means that $$(3\cdot 1+1)^n=256 \implies n = 4$$

Expanding $(3x+1)^4$ gives the $x^2$ term as  $\binom{4}{2}\cdot(3x)^2=54x^2$. Hence, the answer is $\boxed{54}$.