I commend the OP for their large aux fuel tank ... good for them! Some of the world's most expensive RVs have outstandingly large fuel tanks to provide tremenduous TRAVEL RANGE.
The main reason for a large fuel tank is range -> so one doesnt' have to stop and mess with gassing up any more than absolutely necessary.
We once had a Dodge B200 3/4 ton van that we converted into a camper. I hate gassing up so much that I installed a 40 gal aux tank on it for a total gas capacity of around 65 gallons. At it's 18 MPG ... you do the math on what our travel range was.
I like to stop when I have to go potty ... not when the RV wants to drink.