Why not run portable generator off vehicle's fuel tank?

Nothing.. Your missing nothing

The op states that for his use the Honda is a better choice
To paraphrase
"Why can't i use my vehicle fuel tank, instead of gas cans"

And he doesn't accept the reasons why
Because nobody can point to source and give him a quote

There seems to be a lot of subject confusion in this thread, making it hard to follow.

An Onan or other unit built into a class a, b or c is hardly portable. At least, my idea of portable is something designed to be easily moved, and run on its own outside.

What am I missing?

I think it was explained quite clearly
Motorhomes equipped with a built in generator come with a tank installed with a fuel pickup for the generator, that is separate from the in tank fuel pump system that is in your car truck etc
Your truck car etc.. Is not design for a secondary fuel outlet
Now if you have a motorhome with a built in generator and would rather use a Honda
Feel free to use that fuel line for your Honda

You want a black and white in print verifiable quote
Then write the mfg of your tow vehicle/truck for it

In my OP I said "I'm sure that there is a good reason---" and I'm still convinced that is true. I'm now also convinced that none of you know that reason, either! Hey, I can guess with the best of you! My point was, an Onan draws from the vehicle's gas tank. That is seemingly it's only advantage over the smaller, lighter, quieter, more fuel efficient, and MUCH cheaper Honda, which DOES have the ability to run off of a separate tank, and no, it's not a gravity feed. (and, um, we are talking gasoline here, not diesel or propane) So it seemed logical that at some time or another somebody would have figured out a way to do this unless there was a very good reason not to. And, there probably is, but that is still a mystery to me, but we got three pages of entertaining guesses, so thanks anyway! Don't worry, I won't be hacking into my fuel system--

Around here ethanol mix, is all you can buy, unless you go buy racing fuel
I use to much gas to buy racing fuel
Ethanol is not much problem when your using it every day
It's when you let it set for long periods of time than it clogs things up

Yes mfg use separate fuel pickup for the generator
But i for one, would not be willing to pull the Gas tank out of my truck
to install a new pickup even if i could find a dual pickup to install

The op wanted to know why this wasn't being done
I think it's been fully explained .. why it's not being done, why nobody has done this

If he doesn't want to carry gas cans, if he wants long periods of extended run time
I suggest he research auxilary bed mounted fuel tanks

I wouldn't want to run ethanol gas through my Honda. I only run Non- Ethanol in any small engine except and my boat. Ethanol fuel is horrible on small engines.

2oldman wrote:
All things considered, this is a bad idea.

Done properly with a separate pickup installed in the tank, it should work well with a fuel pump equipped generator. Motorhome manufacturers have been doing that for years for onboard generators.

TNGW1500SE wrote:
Most vehicle tanks are a sealed system. How would you allow air in to replace the fuel you burned? You could collapse the tank in if you didn't allow air back in. I'm sure it could be done but I think you'd be asking for problems.

LOL, how does the vehicle allow air in to replace the fuel you burn?

2oldman wrote:
All things considered, this is a bad idea.

I would agree with this statement.

Most vehicle tanks are a sealed system. How would you allow air in to replace the fuel you burned? You could collapse the tank in if you didn't allow air back in. I'm sure it could be done but I think you'd be asking for problems.