8 years is pretty darn good. Most high end batteries last 6 years or longer. Proper charging maintenance and discharging is key to battery longevity. AGM batteries usually slowly fade away meaning they won't hold as much of a charge as they age. You can always have it tested if you're not sure its dead.
You can purchase what you want but IMO the better batteries use higher quality lead and for the cost you'll end up with a battery that generally has a longer life. So for the most part you get what you pay for. With that said improper discharging and poor maintenance can kill an expensive high quality battery just as fast as a cheap one. The worst part about batteries is there is a huge mark up on the price so you might want to shop around. If you can find someone who can work or purchase from a dealer you might get a break in the price.
Before you purchase you want to make sure the batteries dimensions will fit the mount correctly especially the height and type of the lugs. Several batteries have the same (or similar) dimensions but the lugs might be too tall. If your van was built in Fresno chances are the battery is a Lifeline brand made by Concorde. Concorde also makes a battery called Sun Xtender
which is the same as the Lifeline battery without the extended warranty. Those might be worth looking into but always check the size for correct clearance.
Rolls Surrette, Deka, Odyssey, Trojan and others make good batteries as well.
If you know somebody who owns a auto shop you might be able to find a better deal. Always check if there is an added fee for haz-mat shipping. Some Solar places might be able to put you on to a good deal if you ask. You also might check out Northern Arizona Wind & Sun
to get an overall look at things. A golf course shop or a place that works on golf carts might also be able to give you a discounted price...just shop around. There is always Amazon
Most any shop should be able to change out the battery depending where it's mounted. Mine are on the frame in the back and require unbolting some channel iron and lowering with a floor jack...it's just labor.