PJorgen,
You can't reasonably compare your vehicle AGM starting battery life to the camper deep cycle battery life.
The battery construction details are different and the use and charging of each is different.
Generally speaking vehicle starting batteries are frequently not fully recharged to float after discharging (often due to starting, onboard electronic draw and short trips not long enough to fully recharge). Another problem is the older charging systems employing a voltage regulator which do not charge in stages and may result in chronic slight undercharging (or in some cases overcharging).
These factors lead to sulfation which shortens battery life. The newer vehicles with smart battery charging electronics may do a better job keeping the starting battery properly charged than the older vehicles having simple voltage regulators.
The temperature environment that your vehicle battery survives in is probably harsher than what your camper house batteries survive in and that can shorten the battery life as well (high heat). The challenging operating environment for a starting battery usually results in starting batteries lasting less than 5 years and often less than 4 years in areas with high heat (Southwest for example).
If your camper house batteries are not frequently drawn down below 50% and they are always or almost always immediately fully recharged to float before drawing them down again (maybe because you have a good solar setup) then the house batteries could last a long time maybe 10 years or more. Also keeping camper house batteries in float during storage helps maximize life.
This is just a quick broadly generalized summary. There may be specific details in charging and use that lead to longer or shorter life for either your starting or house battery that are not discussed here.
Edited by ckent323, 06 August 2020 - 06:20 PM.