I also have a 2018 29HFS and the WH in mine is a Suburban model SW6DE. The Installation manual shows a contact for parts and service:
AIRXCEL, Inc., Suburban Division
676 Broadway Street
Dayton, TN 37321
(423) 775-2131 Ext. 7101
Airxcel | AC, Heating & Appliances for Industrial and Personal Use
The manual does not have individual part number information so you might try contacting them.