Re: ford 6.6
that is correct, the lift pump is built into the injector pump. You can feed fuel to the lift pump however....
You do not want to force feed fuel/wvo at high pressure to the lift pump. In a perfect world, you will have your wvo tanks inside the bus which makes them higher than the injection pump and wvo will gravity feed to the lift pump. No aux pump needed.
I also like to route my return line back to the tank. If a return line wasn't needed, then manufacturers wouldn't bother installing them in the first place.
manual valves can be used, but it doesn't cost much more money to buy an electric valve that will switch between two tanks. Think 1990's ford pickup's with dual tanks.
I also route my wvo through the stock fuel filters after it has passed through it's own filter.
Carry spare fuel filters, and kitty litter.
put an aux fuel pump into the diesel fuel line that you can turn on when you run out of wvo. Allows the bus to prime it'self and usually doesn't even require opening the hood even if the engine stalls due to fuel starvation. Alternatively, you can use a primer like the one that is used in marine outboard applications. I install one of these on every diesel vehicle i own. Makes starting the engine after running out of fuel a snap! If you run wvo, you will eventually starve your engine for fuel.
Quis custodiet ipsos custodes (who will watch the watchmen?)