Contact your coach MANUFACTURER and explain to them that your belts do not fit well. They are required by Federal law to provide you with belts that fit, regardless of how old your coach is as long as they provided the original seat belt/shoulder harness. They will GIVE you a seat belt extender that goes between your seat buckle and your seat belt buckle. This will change the geometry of the seat belt in the unit so the belt does not ride high on your neck. These extenders are usually available in 2 or more lengths and are primarily designed for heavy people or well endowed females, but are available for all. Tiffin gave me one for my driver seat when I asked (somewhat forcefully).
I was a Licensed Safety Engineer for many years and taught about this to police, health workers, medical personnel and others. These extenders are also available at your local car dealership parts counter for their brand of cars and are free as well.
You can look online and find seat belt/shoulder harness covers. They are made from a lot of different materials. I bought a set that are sheep skin lined. They are very comfortable and solved my problem. You can also find them at most Autozone stores.
Tony & Ruth........... FMCA#F416727
Newmar Dutch Star 4320, Spartan MM Chassis, Cat C9, Jeep Grand Cherokee Hemi, Blue Ox Aventa LX, and Brake Buddy. TST 507 TPMS