Our 1995 Chevy Roadtrek 190 Versatile is the last year of the old style Chey van chassis with the last year of the TBI 350. We have been getting 10.5 to 11 MPG and also had hot soak vapor lock problems. After some troubleshooting I found that it had low pressure. We replaced the fuel pump which cured the vapor lock and I was pleasantly surprised to find that the fuel mileage jumped to 12 to 12.5 MPG. That may not sound like much but it's a 14% increase so I'm quite happy. The van also has one of the old school pellet catalytic converters so now I'm thinking about upgrading that to a newer higher flow converter to see If I can get up to 13MPG.
My guess is that 1995 5.7 throttle body setup is very similar to the 4.3 TB we had in a 1989 S10 Blazer. I was pretty unimpressed with the TB injection although it was typical of the technology at the time. TB infection is really just a carb setup with fuel sprayed in at medium pressure, usually on a the throttle plate. While it can meter fuel a bit better than a carb, they normally don't use an atomizing venturi like a carb does, so fuel dropout can be and issue. Our 4.3 also had a single plane intake manifold that gave very poor fuel distribution. It was very obvious of the problems when I would pull spark plugs and they showed very large cylinder to cylinder balance based on the color and amount of deposits. I did check good and even on compression across the cylinders and didn't use any oil, so almost certainly mixture and distribution.
Unfortunately, there isn't a whole lot you can do about the fuel issues unless you change the setup to port injection which is neither easy or cheap to have done. A hard core DiY person could put a a later TPI setup on it and do the right tune to match the van, and I would guess the mileage would increase quite a bit. I put a modified TPI based, programmable, setup on a twin turbo 340 CID Dodge I built after running a finely tuned carb setup and the mileage went up a bit, but not a lot, as the carb and intake were very well dialed in. The 600hp, 3800# vehicle was up to about 22mpg compared to a factory 340 carb setup, no turbos, that got about 12mpg.
There used to a be a modified throttle body, that ran the fuel through a venturi like the carb, but used the stock medium pressure injectors, but I doubt they are still available. You might be able to find an aftermarket manifold that does a better job, though as they would probably be a drop in with a carb manifold and adapter. Some thing like an Edelbrock Performer dual plane is a very good setup for efficiency with good balanced distribution.
Changing the catalytic converter may help some, but may not also. Sometimes changes like that don't really have an affect on the fuel input, but would probably increase high load performance.
Pulling the spark plugs to look at them is often the best place to start to get a feel for what kind of things may or may help for mileage. Even at best, I wouldn't expect to get much beyond 14-15mpg on the high end with the pre LS engines like the 5.7. On an older vehicle, gaining a bit of increase is probably cost prohibitive in most cases.
