It has a reasonably new oil boiler (~10 years old) with an oil tank in the basement next to the boiler that appears to be around the same age as the boiler.  

Around my area of North Central Texas, a standard tank based septic system is $2500-$3500. 

*Has 30+ yr old water tank that works fine.

After poking at the water with a stick for a bit, I confirmed there are are at least two four inch long orange fish in this tank. 

@Jay Hinrichs is it common for RV parks to have holding tanks?

Possibly be converted for storage or a small rec room ( not sure if it's really worth it.)Both Have a separate electrical and gas meters but would share one water heater.

Are you saying that a tenant will rent the home and put in their own furnace and hot water tank?

You can't have any holes in the screens, you can't have more than 1 person per 50 sq ft of bedroom space, no flexible pipe from the sink drain and no plastic water supply to any faucets, GFIs in basements and garages and within like 10' of the sink, no flexible pipe to hot water tank or furnace, no nail holes or non-fire rated foam crack filler in the garage wall between the garage and the house because fire can spread faster from the garage to the house... 

. :-)Marketing would be handled by Jerry Puckett.The sharks from "Shark Tank" would be my PMLs/Finance Division.

Then during her pre-move, walk-through she kept expressing worry that the basement and garage didn't have enough storage for their stuff...