We had one of those safety straps on our Viking PUP for our holding tank also. But the tank itself was actually supported by the frame. When ours broke after 7 yrs DH just replaced it but there was no real reason for it to be there, like you said, maybe during the assembly. It just gave him peace of mind to replace the part.
Joe and Mary Anne
1 18 yr old DD
2014 Ram 2500 Diesel Laramie Mega Cab 4 X 4
2011 Jayco Jay Flight 26RLS-Sold
2014 Jayco Eagle 298 RLDS