Looks like my replies are getting blocked so I'll try from scratch.

Sometimes timing can be amazing. I have a van here that has an Interstate 96R battery installed for the engine. It fits! It is shorter that the group 41, closer to the group 42 as used in the air cooled Vanagons and other VWs like the Type 1.  Doing some research there is also a group 91 that looks to more closely replace the group 41 normally used in the Vanagon.

Looking for AGM I was only able to find some in the group 96R. Those looking for AGM option this should be a good choice. It may be a better alternative for the house battery also. Although not a deep cycle design it may hold up better than the standard group 41. If using one of the smaller AGM batteries for the house load there may be benefit to using the group 96R AGM for the engine to better balance the voltages when the batteries are combined for charging. One thing to note is that AGM batteries at rest sit at slightly higher voltage than flooded type batteries. When using voltage operated combiners this high voltage will keep the batteries combined much longer. This actually has some benefit but the difference in operation is noticeable.

There also some Lithium batteries in the 40 to 50 A/H range that are becoming quite cost competitive with AGM. I will look to try one of them soon.

https://www.amazon.com/LiFePO4-Phosphate-Rechargeable-Off-Grid-Applications/dp/B0BDV7MYHR/ref=sr_1_7?crid=3FJAH2S752MV5&keywords=50%2BA%2FH%2Bbattery&qid=1691795489&sprefix=50%2Ba%2Fh%2Bbattery%2Caps%2C150&sr=8-7&th=1

Dennis