I agree, slowing down will save the most fuel. For example in any speed zone over 60 we drive 62-63 even if the limit is 75! Let the guys speeding by take the risks and burn more fuel! In any speed zone of a lower posted speed we drive that speed (45, 55, etc.)
Do replace your dirty air filter as often as needed because a dirty filter can damage the engine which is more costly and if it does use less fuel after a filter change you are ahead even if you think you aren't.
Folks complain all the time and with reg. gas and diesel being really high we all want to save money. But, towing or not, not running the generator or not, or not going across mountains will only save a small amount of change versus the hassle & discomfort of not doing all that. Just keep the engine in top shape, drive slower, try not to drive on windy days, and it will use the amount of fuel it does.
There is no RV of any type that will not cost you to own and use it. The real truth is...if we can' afford it then we shouldn't have it. Do the best you can to maintain the rig to top specs then enjoy it. Or sell it and decide what to do with the money you save.
Another option is to buy a trailer which doesn't have an engine and transmission to maintain but you will still have to maintain and run the truck to tow it.