RCM payable in freight charges missing

RCM liability not declared, that means same is not paid and therefore it's ITC also not availed in 18-19.

Assuming you reported above liability, paid the same and availed it's ITC in next year 19-20, you have to show all this in 19-20's GSTR-9 (it's reporting is Not to be done in 18-19 GSTR-9).

If expense attracting reverse charge was booked in an earlier month and rev chg liability was not declared in that month then yes, you need to pay 18% interest.