I went a took a closer look underneath. The tires are low and require replacement, so I'm not able to thoroughly inspect the undercarriage until it's jacked up.
It does have a drain pipe with spigot for grey water that extends to the rear corner, however I can't see any grey water tank, possibly it's installed somewhere inside.
The frame is approx. three feet wide with ~1/2 of the three feet is the driveshaft and muffler. The rear of the frame has a 25 gallon fuel tank and the front the engine and transmission.
It appears the only space for a black water tank would be somewhere in the middle of the frame on the half without the driveline and exhaust and muffler.
If installing a black water in the middle of the frame would present a problem with the inlet and the outlet (drain). An RV toilet would not have a straight drop to the black water tank which is usually neccsary when installing a RV Toilet. However I remember reading about RV black water tanks that aren't installed directly under a RV toilet.
Though most RVs do have their RV toilet positioned directly over the black water tank for a straight drop there is an option to install a RV toilet anywhere in a RV by using a macerator and pump.
Some macerator are installed separately while others are installed in the RV toilet.
The macerator churns the effluent and pumps it to another location such as a black water tank and/or sewage or septic tank.
I have considered a low flow flush toilet that uses around 1.6 gallons per flush, where a RV toilet uses ~.5 or less gallons per flush. I think a few low flow flush toilet may use closer to 1 gallon per flush. However it's still uses too much water compared to a RV toilet that typically uses ~.5 gallons or less per flush.
Macerators and RV toilets with macerators can be expensive.
However I'm not certain if I would need two macerators as I would need one to pump effluent from the RV toilet into the backwater tank and another pump to push the effluent out of the black water tank.
I'll probably need to look at some installations to figure out a common design.
One problem with installing a blackwater tank in the middle of the E-150's frame is getting the effluent to drain, so it most likely would require a pump of some sort to pump the effluent out of the black water tank.
There are portable macerater units that you connect to the blackwater tank's outlet, churns the effluent and sends it through an attached ~1 inch hose.
RV Holding Tanks