The Costco Group 27 RV/Marine batteries aren't "true" deep cycle batteries, but rather are a hybrid type with starting capabilities. However, in our older Roadtrek which has room for only a type 24, we use the same kind of battery (95 Ah) with very good results. We have a small solar panel on the dash that keeps the house battery fully charged when the Roadtrek is being stored.
Regarding the Costco type 27 battery cited by Thrifty Dutch, our starting battery needed replacement, and this battery was the least expensive, and starts the Roadtrek easily in all weather conditions. As we use a 1500 W inverter connected to the engine battery to power the microwave oven, this type of battery serves us well. (We run the engine when the microwave is being used.)
Another point: Costco here in Canada unfortunately doesn't have as extensive a line of automotive/RV batteries as in the States.