That particular dealer's answers do not match with Mercedes UK EQV official brochure and configurator which make no mention of towing capacity, possibly due to range loss from aerodynamics. Some aftermarket companies do offer a hitch though -
www.rameder.eu
.. however if warranty is explicitly affected then personally I would consult with a specialist legal expert and a consumer advice body especially if you bought from that particular dealer and towing is essential plus if you won't have room in the van.

Brink towing hitch Towing hitch - year of make - | Rameder towbar
Brink towing hitch for at the best price. Buy towing hitch with 2-hole flange ball for your now.

.. however if warranty is explicitly affected then personally I would consult with a specialist legal expert and a consumer advice body especially if you bought from that particular dealer and towing is essential plus if you won't have room in the van.