Any thoughts about intake and exhaust systems?  I'm considering one for my 87' Airstream 345 with the 454 powerplant. I've heard that folks who have anted up for the Banks kit swear by them but they are expensive - about $3500 worth.  Gibson is supposed to render simalar results for less.  Are there other vendors out there? What about home brewed systems?  
     

First off, welcome to the site. I have a 1986 345 and have done some work in this area. For intake, I've rebuilt my stock QJet. I've also replaced the distributor and installed a heavy duty coil from NAPA along with Accel 9000 High Temp plug wires. I've also added a "backyard" Banks system by using Heddmen headers for the P30 chassis along with 2.5" collectors into an H Pipe, then two high flow mufflers and a complete 2.5" system.

I'm very happy with the results. We get about 8 mpg towing a 1700# and better than that when not towing. A few others have done more upgrades for cold air intake and ignition (Jacobs, MSD, etc)...which is an area for improvement on the stock 454.

What mileage were you seeing before the modifications?  I am getting 8.7 at 65 - 70(actual) not towing.  My speedometer reads about 7mph fast ( I think they all do) so my calculations are off by that much.  I'm using the speed shown on a GPS which is absolutely correct but unfortunately does not have an odometer.  Based on my speedometer error which is roughly 10%, I assume that my odometer is clocking 10% more miles than actually traveled.   In that case, my coach has about 60,000 miles on it instead of the 70,000 (rounded) shown.  Wouldn't it follow that I'm covering 10% fewer miles than the odometer readings that I've used for mileage calculations?  As a result, I'm not really getting 8.7 but something less.   Sometimes ingnorance IS bliss. Maybe there is a math lover out there  who can  come up a formula to correct the odometer mileage by the percentage error?   Do y'all see speedo errors?  Are we all victims of GM"s desire to sell us a warm and fuzzy feeling in the 80s? 

Oops.  I forgot to mention that I have installed a Gear Vendors under/overdrive which lowers my RPM considerably.
While I'm at it (again) I put in a 160 degree thermotat.   Originally, a 190 degree t-stat was installed to help emissions.
My unit runs much cooler and will last longer as a result.

I see about 5 mph faster on the speedo than I'm actually travelling. Never thought it was the speedo, always assumed the tires where changed, etc.

We get about 8.0 - 8.5 when towing on the stock 3 speed, but we tow slower; about 55 - 60 mph.

It seems that every vehicle I check with a GPS reads faster than actual with 5 MPH being the average error.
Coincidence?

Try replacing that Qjet with edelbrocks 650 thunder series AVS an X pipe & walker dynomax mufflers

My 87' has TBI.  Isn't that preferable?