Vehicle starting batteries, both flooded and AGM, have large numbers of thin plates and are designed to produce large current for a short time and be immediately recharged. Some when totally dead are hard to revive especially when dead twice; and they will often not last long. A good rule even for apparently good batteries is to replace after 4 winters as the chance of failure on a cold morning is high during the fifth winter.
Proper house batteries, both flooded and AGM, are "deep cycle" having fewer but thicker plates designed to put our small currents for a long period before recharge. From the get-go they lose capacity every time they are cycled. Manufacturers have various tests using standardized tests (rate of current draw over time periods) to determine the number of cycles (typically 500) before the battery is effectively useless. Battery usefulness is extended by keeping discharges shallow and recharging as soon and as often as possible. The user decides when to replace when capacity seems to be insufficient for the service needs. I am personally knowledgeable of a large bank of quality deep cycle house batteries lasting over 10 years on a boat.
Re post 7 above: I fully agree as I am also fused at 80 amps from the van battery to the combiner. Starting current is likely to blow that fuse so it is best to connect the two and see if over a period of a few minutes the van battery can receive enough charge to start after disconnecting the bypass. This can be especially effective if the van can be plugged in so a shore charger can boost both batteries. Many of us have house batteries near the seats and reasonable length jumper cables could allow connection out the open door to the under hood jumper points. Although more difficult shorter jumpers can be used to go from the house battery to the van battery under the floor.