stock-rma/rma/report/report_deliveryslip.xml

<?xml version="1.0" encoding="UTF-8" ?>
<odoo>
    <template id="report_delivery_document" inherit_id="stock.report_delivery_document">

        <xpath
            expr="//table[@name='stock_move_table']//th[@name='th_sm_quantity']"
            position="before"
        >
            <t
                t-set="has_rma"
                t-value="bool(o.move_lines.filtered(lambda m: m.rma_line_id))"
            />
            <th t-if="has_rma" name="th_sm_rma_line"><strong>RMA</strong></th>
        </xpath>
        <xpath expr="//table[@name='stock_move_table']/tbody//td[2]" position="before">
            <td t-if="has_rma">
                <span t-field="move.rma_line_id" />
            </td>
        </xpath>

        <xpath
            expr="//table[@name='stock_move_line_table']//th[@name='th_sml_quantity']"
            position="before"
        >
            <t
                t-set="sml_has_rma"
                t-value="bool(o.move_line_ids.filtered(lambda m: m.move_id.rma_line_id))"
            />
            <th t-if="sml_has_rma" name="th_sml_rma_line"><strong>RMA</strong></th>
        </xpath>
    </template>

    <template
        id="stock_report_delivery_has_serial_move_line"
        inherit_id="stock.stock_report_delivery_has_serial_move_line"
    >
        <xpath expr="//td[@name='move_line_lot_qty_done']" position="before">
            <td t-if="sml_has_rma">
                <span t-field="move_line.move_id.rma_line_id" />
            </td>
        </xpath>
    </template>

    <template
        id="stock_report_delivery_aggregated_move_lines"
        inherit_id="stock.stock_report_delivery_aggregated_move_lines"
    >
        <xpath expr="//td[@name='move_line_aggregated_qty_done']" position="before">
            <td t-if="sml_has_rma">
                <!-- TODO: To be implemented.
                It will require de extension _get_aggregated_product_quantities
                -->
            </td>
        </xpath>
    </template>

</odoo>