Yes, you're 100% right. They are poor quality. They just don't last.
Deep cycle batteries are not meant to be used as starting batteries. They can't withstand
that shock load. They are meant to be slowly drained to half, an then slowly recharged.
They'll do the job, but won't last.
Will, the Nautilus deep cycle marine group 27 batteries last 10 - 11 years in my 1655, and sometimes sit for months without starting or using a charger, then lights off the engine with no problem. Kinda pricey at CTC, though. The last time I started the 1655 was in the winter, it had sat for 14 months with an 9 year old battery. The battery finally froze last winter after sitting another year without being recharged. At 10 years. I'm happy.