you probably don't have a shut off valve on your rear heater, which is fairly common. Get yourself a few hose connectors, they look like this
and a few feet of hose. (2 pieces, one to loop the hoses and one to loop the heater itself) with a cold engine remove your radiator cap to release any pressure. Leave the cap off, remove the hose clamps from your heater connections, now remove the first hose (it might be baked on there due to time-heat etc. and be easier to cut off then pull off, what helps to pull it off is warming the rubber up first with a heat gun, hairdryer, torch or what ever you have.) and quickly connect the extra pieces of hose and hold it up. (make sure you got the right size
)You want loose to much fluid if you hurry up. maybe have a second pair of hands handy. Remove the other hose and connect the two. Now remove or move your "hated heater".
If you wanna get rid of the heater all together, just loop from one inlet to the other in your engine compartment. Looks like you could just use a hose connector and some hose clamps on these 2 hoses.
I would not worry about removing the hoses under the truck, just loop them together too, then they are there and ready to be used again if you decide down the road that you do need a heater.
Tip: if you park your Bus on a downhill slope to do this procedure your heater in the back will probably be higher then the radiator and you want loose that much fluid.
When all done fill your rad with 50/50 (thats for us in the cold Canadian north, you might be ok with 75/25 mixture) water / antifreeze and tighten the radcap, fill the expansion container to the cold line and start your engine. Check for leaks. after the first drive you might need to refill some antifreeze mixture.
Good luck and happy converting.
add on: the most fluid loss will be coming out of the heater core, so if you are able to remove the bolts that hold the heater do that first. make sure you got them all so that the heater is free to move. Then when you disconnect the hoses simply put the heater on its back so the inlets are facing the ceiling.