from sale_views:

record id="sale_order_view_search_inherit_quotation" model="ir.ui.view">
	​field name="name">sale.order.search.inherit.quotation	​
	​field name="model">sale.order	​
	​field name="mode">primary	​
	​field name="inherit_id" ref="sale.view_sales_order_filter"/>
	​field name="arch" type="xml">
	​	​xpath expr="//filter[@name='filter_create_date']" position="replace">
	​	​	​field name="campaign_id"/>
	​	​	​separator/>
	​	​	​filter string="My Quotations" name="my_quotation" domain="[('user_id', '=', uid)]"/>
	​	​	​separator/>
	​	​	​filter string="Quotations" name="draft" domain="[('state','in',('draft', 'sent'))]"/>
	​	​	​filter string="Sales Orders" name="sales" domain="[('state','in',('sale','done'))]"/>
	​	​	​separator/>
	​	​	​filter string="Create Date" name="filter_create_date" date="create_date"/>
	​	​/xpath>
	​/field>
/record>

and this is my so_custom_views, I want add custom filter after "sales" filter from above:

    record id="sale_order_view_search_inherit_quotation" model="ir.ui.view">
	​	​field name="name">sale.order.search.inherit.quotation	​	​
	​	​field name="model">sale.order	​	​
	​	​field name="mode">primary	​	​
	​	​field name="inherit_id" ref="sale.view_sales_order_filter"/>
	​	​field name="arch" type="xml">
	​	​	​xpath expr="//filter/field[@name='campaign_id']" position="after">
	​	​	​	​field name="proofing_status" />
	​	​	​	​filter name="proofing_status" string='SPK not Proofing' domain="[('state', '=', 'done')]"/>
	​	​	​/xpath>
	​	​/field>
	​/record>

but there's error:

Traceback (most recent call last):
	​File "/home/desi/odoo-13.0/odoo/addons/base/models/ir_ui_view.py", line 669, in apply_inheritance_specs
	​source = apply_inheritance_specs(source, specs_tree,
File "/home/desi/odoo-13.0/odoo/tools/template_inheritance.py", line 234, in apply_inheritance_specs
	​raise ValueError(
	​ValueError: Element 'xpath expr="//filter/field[@name='campaign_id']">' cannot be located in parent view

even I also already tried using:

xpath expr="//filter[@name='sales']">