Ford V10 gradual reduction in fuel economy

[whemme]

I have a 26' Class C on a 2000 Ford E450 chassis with V10 engine now with 115,000 mile on it. I used to average 8.5 mpg pulling a 3400 lb toad but over time the economy has deteriated to the 7.5 to 8.0 mpg range. My Ford dealer says that as long as I am not getting any check engine codes that my oxygen sensors are still OK and not causing the reduced mpg problem. But I still wonder if the oxygen sensors might be causing the problem. Anyone else with experience with this issue?

[whemme]

PartyOf Five wrote:
We add a couple cans of seafoam periodically to clean the inserts but I'll be curious what you learn also.

Not sure what you mean by inserts. Do you mean injectors?

[PartyOf_Five]

We add a couple cans of seafoam periodically to clean the inserts but I'll be curious what you learn also.