I would contact Tiffin and get the supplier contact information. I'm sure they will have what you need or can make it for you at a reasonable price. Do it once and do it right. Any modification would weaken or at least look funny. I just noticed from your picture that the tube is oval shaped. The supplier would be my only option if it were me.
2018 THOR Chateau 35SF
Two Labs, Bugsie & Blondie