We also have an older (2004) Ford E350 based MH. Low end I guess. Rather than an extra battery isolator it uses the built in Ford auxiliary battery charging circuit which consists of a fuse and relay in the engine fuse box. Look up the details under fuses and relays in the Ford manual.
When I had engine charging of house battery shortcomings I found that the charging voltage was a couple of volts below what the engine battery was feeling. It had only 12 gauge wire but that wasn’t the main problem; it was a connection with a half volt loss, a long wire to a poor battery disconnect switch, rather than a relay, losing another volt.