If you are not paying around $400 for an 8D, it's not a deep cycle. A $150 8D is a starting battery with thinner plates.
Possibly , but what you are paying for is the LEAD in the battery , the case and spacers are cheap 50c a pound plastic.
The yachty markup $400 for $150 worth of lead in a different plate thickness is partially caused by by the lower production of deep cycle batts , VS the huge number of 8D for Trucks, Coaches, Cranes, Ag & earth moving etc.
Since many of these commercial uses DO require more than just a thin plate start batt ,(like a 31) the 8D usually is a fair compromise , but certainly not for repeated real deep cycle work.
Many OTR trucks run lots of lights with an Espar cycling of and on for 12 hours , sometimes with a TV , and lap top , and bunk heater. No place for just a start batt , but the cheapo 8D seems fine with the service.
The 8D in this service are seldom deep discharged , and with a 10 hour charge time , even an imbecile 1 wire V regulator can bring them to 100% , so they live long and prosper (3-4 years).
Check the CCA as well as the 20 hour rate for your next batt endeavor , as simple and cheap can work for many.
IDEAL solutions are fine IF you fully use them , otherwise a waste of currency.