A 15 watt solar panel behind the windshield laying flat is only going to produce about 5 watts.
Waste of money.
Looks like the diesel has a warm air intake. Not good for fuel economy unless you are operating in below freezing temperatures. Looks to be not cold there.
High 20s definitely possible, if not more and room for improvement too.
1984 chevy suburban, custom made 6.5L diesel turbocharged with a Garrett T76 and Holset HE351VE, 22:1 compression 13psi of intercooled boost.
1989 firebird mostly stock. Aside from the 6-speed manual trans, corvette gen 5 front brakes, 4th Gen disc brake fbody rear end.