There were lots of two-tank E-Series in those years. Should be some parts availability used and probably new. There are also aftermarket Tank Select Valves for around $100. Simple as a gOOgle search will probably turn some leads.
Wheel covers, 16.5"... When we bought our 1984 Class C/E350, it had three Hub Caps. They weren't simulators and they didn't bolt in place. Just old timey bang-in covers. I recycled the three and painted the wheels.
E-Series didn't stay with the 2-tank setup all the way to the end of the years they produced the design. 1975-91 as I recall. You may be able to replace the current rear tank with a bigger late model tank and be done with it. That's given the coach builder didn't block it in with holding tanks etc. and you can make or access adequate mounting points.
