Planning a Lake Powell Trip, any tips?

My friends and I are in the process of planning a trip to lake powell sometime next july. We are planning on renting a house boat from either waheap or antelope point marina, and being on the water 4-6 days. I dont even own a boat yet but plan to buy one in the next few months. I plan to tow that there, we are from SD and have about 20 hour drive. Just looking for any advice or tips.

Any must haves on the houseboat?
Any tips on when to book the houseboat? Sounds like they start running specials in October, we just really dont want to be to late and be stuck with dates that we can't make work.
Anyone have any recommendations on a specific boat for 10-12 people?
Is July a decent time to go?
Things to be sure to bring?
Things to be sure to do?
Things to be sure to see?
Any tips for towing a boat that far?
I am somewhat nervous on the fuel situation with the wake boat, I know the lake is huge, and we may end up camping pretty far from the marina, will a guy just need to watch your level carefully and go back to the marina to fill up daily? Bring some spare too probably? Anyone ever brought a transfer tank on a houseboat? Assuming they probably wont allown that.

Any and all help/tips/advice would be greatly appreciated.
We can't wait to go, have never heard of anyone going and not having an incredible time.

Thanks!