While we do not currently offer custom-made holding tanks, we can assist you in finding a suitable grey water tank for your Haulmark motorhome. We carry Ameri-Kart products, a reputable manufacturer of RV water and waste tanks, which may have options matching your specifications. You can browse available Ameri-Kart tanks on our website. Additionally, we recommend using our Tank Finder tool to help identify tanks that fit your required dimensions. If you need further assistance or have specific fitting requirements, please provide detailed drawings or additional information so we can better support your search.