I don't know about which toggle might work, sorry, but someone here will. Also try the Open roads forum on electrical stuff as noted above and Ford Truck Enthusiasts forums. All good for gathering info. My isolator (voltage sensing relay, actually) does NOT have a manual option. It is automatic but works great. You may want to read more about it on Amazon (including reviews), but there are lots of ways to isolate and switch batteries.
Pure sine wave power works just like AC power and won't heat up or damage electric motors or transformers (chargers). Modified sine wave inverters are much cheaper and put out power plenty pure enough to run most electronics (refrigerators, microwaves, kitchen appliances, etc). BUT, i'm no expert. I do know for sure that fridges and microwaves and coffee pots are ok on modified sine wave but not sure about other things.
Read the FAQ here towards the bottom of the page, very helpful site: http://www.donrowe.com/
Do you have 2 batteries already? You should on a diesel, one is probably under the hood and one along a frame rail underneath, i think. Someone will chime in soon and answer this for you, no doubt. what year is your van?