Use Short Jumper Cable to Boost Automotive Battery
This quote is attributed to George Clements, a former employee of Home & Park Motorhomes:
"In an emergency you can boost the automotive battery off the isolator located in the engine compartment. You can do this using booster cables."
This is done by connecting a jumper cable from one outer connector on the isolator to the other outer connector on the isolator.
On most vans, there will be a fuse or a breaker on the cable from the isolator to the house battery. If you try to start your van while the jumper cable is attatched to the two outer connectors on the isolator then you will probably blow the fuse.
To avoid that, use your house battery to 'charge' your discharged chassis battery for 15 to 30 minutes by connecting a jumper cable from one outer connector on the isolator to the other outer connector on the isolator.
Then, remove the jumper cable before trying to start the motor.