Our tanks hold 75 gallons fresh and 55 gray. I'm not worried about the 45 gallon black in that short of a period (also, the University has restrooms (no showers) set up in the area).
The amount of water needed simply depends upon how much you conserve. If you use the "submarine shower" technique of turning off the water while you wash and then rinse, you don't run water while brushing your teeth, use a dishpan to wash dishes and a few other things of that type, you should be easily able to make do with that amount of water. We used to dry camp in a parking lot while at square dance weekends and we had 40 gallons of fresh water that we could make last, even with each of us showering twice from that water. Done right, a shower can be taken with fewer than 5 gallons of water per person. We used to figure that it took about 3 gallons.
It will be pretty warm and we will have to run the 5.5 Onan all three days full time. Will this be a problem? Do they overheat? Should we shut it down occasionally? I'm also wondering what amount of gas it will use running two AC's. Someone told me about 12 gallons a day, but that seems low.
We have run our 5.5 Kw Onan supplying two air conditioners for as long as 4 days without stop. The book does recommend that you shut down and check the oil every 50 hours, which would be roughly two days. Since the Onan also has a low oil, safety shutdown, I never bothered to do that, but just checked it at the start of the run and again at the end. Our manual suggested that it would burn about 1/2 gallon of fuel per hour of run time, but we found that it seldom used that much. On our time of running for four days without stop, I filled the fuel tank of the coach (75 gallons) just before I parked it and after four days of use it took about 30/35 gallons to fill it back to top.
Be sure that you keep the access pannel to your genset closed securely at all times when the engine is working as that is required for it to get proper ventilation. The Onan has a blower that pulls cool air into the engine and then blows it out of the bottom but if you open that door it will not operate properly.
The need for an exhaust pipe extension is somewhat dependent upon the length of the existing tail pipe and also your proximity to other RVs. You do need to have enough room around the coach for ample air flow through, under and around it to disburse the exhaust gases from it. The closer together that RVs are parked, the more you may need something of that sort.