From the 3d catalog parts viewer on the Winnebago website it shows a drain valve under the bed in an area more towards the center of the RV. Close to the water pump but it looks like there is a divider between the water pump area and the drain valve area.
If you have not used the 3D Service Parts Lookup catalog it is a great tool. It can be found on the Winnebago website on the Owners Resources page under helpful links.
The things you own control your life.
2021 Winnebago Vista 35U towing a 2022 Jeep Cherokee Trailhawk
2010 Cadillac SRX 2003 Itasca Sunova 30B