How to set a range in excel VBA from a value to that values row index and the last column in that row?

Of the things you have allegedly tried...

Range(RValue, Cells(RValue.Row, LastColumn).Column)

gives a Type Mismatch error, assuming that LastColumn is a range

Range(RValue:cells(RValue,LastCol))

just gives a compile error because of the colon :

If you want to include a colon in a range definition then use it within a string e.g.

Dim x As Range
Set x = Range(RValue.Address & ":" & LastColumn.Address)

You are actually in the right space with

Range(RValue, Cells(RValue.Row, LastColumn.Column))

What makes you think that it doesn't work?

Sub demo()

    'The "source"
    Dim RValue As Range
    Set RValue = ThisWorkbook.Sheets(1).Range("A7")

    'The first cell in the last column used
    'This can be determined in a number of ways
    Dim LastColumn As Range
    Set LastColumn = ThisWorkbook.Sheets(1).Range("E1")

    'Replicate the Value in Range RValue through from RValue to the Last Column on that row
    ThisWorkbook.Sheets(1).Range(RValue, ThisWorkbook.Sheets(1).Cells(RValue.Row, LastColumn.Column)).Value = RValue.Value

End Sub

Alternatively you could have used OffSet

ThisWorkbook.Sheets(1).Range(RValue, RValue.Offset(0, LastColumn.Column - 1)).Value = RValue.Value

I will also repeat my advice about being explicit about ranges - don't just use Range, that applies to the Active sheet in the Active workbook - and that can easily change with a single click of the mouse - Users will do that. Be explicit - state which Workbook, and which Sheet, or use Withe.g.(Note the periods before Range and Cells!!)

With ThisWorkbook.Sheets(1)
    .Range(RValue, .Cells(RValue.Row, LastColumn.Column)).Value = RValue.Value
End With