Now what?

I'm collecting parts to make the single exhaust on the RV into dual exhaust. Probably will end up about $175 was hoping for $150 but it always costs more than expected, takes longer to do and encures several problems in the installation.
Normally dual exhaust reduces exhaust gas back pressure, increases horsepower and increases gas mileage by 1-2 mpg. I don't expect that much with the RV because of the weight (6700#'s including the trailer and bike of 1000#'s) and aerodynamic drag. It's a big box going thru a fairly dense fluid (air) and drag increases exponientially with mph.
That's why I've slowed down to 55-60 mph.
Also put an air dam on the front under the radiator that pivots in case we hit sometime (tire shapenal or speed bumps). Hope this gives about a tenth of a gas mileage improvement.
From here on out major bucks would have to be spent to increase HP and MPG with little payback.
I start with the cheapest. Tune ups, free flowing air filter, open up the intake track and allow cooler inlet air. Then check the intake manifold, look for vac. leaks, check computer codes for failed sensors. Then the lengthier process of reducing any weight. I then remove as much outside, top and underneath air drag points as possible. (Mirrors can cause a .5 mpg loss!, dirty vehicle .1 mpg)
Then go for the exhaust. This one has two narrow 2" exhaust pipes going into a restrictive collector Y pipe that is only 2 1/4" down stream with a catalytic converter and a very quiet (restrictive) muffler and even narrower bends. I'm taking one whole bank of cyl's, 4 of the 8 and adding it's own exhaust system with less restritive cat. converter and muffler. This will add 25 lbs but as stated before "should add" HP (10?) and MPG (.5?)....
More after our next trip.
Pic is Linda drying out a cooler at Monahans Texas Sandhills State Park (& 105 degrees)