don:

already many good posts, but here are three sugestions:

1. the place to start electrical checks is at the battery itself, not the
alternator. check voltage across battery terminals with no load. then
start the engine and see if volatage increases, still at minimum load
(i.e., no lights). if yes, there is some charging action. see what happens
with more load, such as lights. even 12.75 - 13 V under nominal load
should get you home, assuming a good battery.

2. if it comes down to driving on battery only, obviously the load should
be kept to a minimum. daylight driving with no lights and so on is
obvious. also, COMPLETELY (as in every single connection) electrically
disconnect the alternator from the van to further conserve electrical
energy.

3. new batteries usually come off the shelf only partially charged, so it
would be prudent to have any new batteries charged up before hitting the
road. if it comes down to 2. above, why not have, say, 4 or more
batteries; and maybe hook two up in parallel (positive to positive) using
a pair of booster cables. an inexpensive hydrometer, and/or monitoring
voltage across terminals, would help warn of approaching low charge
condition, and time to drop in a fresh battery for parallel connection,
and so on. keep one battery in reserve for emergency starting and to
extend range if necessary. a little experimenting will tell how far you
can go this way, but it would be a looong way, with plenty of charging
opportunities in between.

dan