First I would recommended that what ever house battery and isolation switch you have is disconnected from the starter batteries. I would just unbolt the bridge cable at the iso switch and tape it off just to be sure there is no connection between the "van" and the "house/camper"
Then I would charge the new batteries fully with a plug in charger. I'd probably do this with the battery ground cable disconnected from the van. This is to just charge the batteries so they aren't damaged from low voltage. Batteries probably weren't cheap.
At that point you would have fully charged batteries, and the "van" separated from the "house". Then I would confirm clean and tight grounds, battery posts, positive cable connections at starter and alternator ect.
Finally reconnect the batteries and see what voltage drop looks like. If it's still seems low then we move on to other possible issues.
2005 SMB RB 4x4 6.0 PSD
A rocket on the pad is safe,
but it's not what rockets are built for.