How many showers and water use is related to the size of the holding tank. Last year we had 100 gallons, the showers lasted until the last day. In fact all of the tanks were full by the end.
You can dump the gray water tanks anywhere (on the ground), all it is is shower/sink water, then fill up with water again. The black water tanks have to be dumped, there is a dump station at the facitlity (LTM). Make sure who you rent from includes additional checmicals incase you need to replenish the tanks mid week.
Ask them if they have a unlimited use rate for the generator. Last year we didn't have to use the gen at all, in fact used the heater. Make sure they fill up the propane tank. They SHOULD supply enough of everything to last the length of your trip.
*Please note. There is NOT an electrical connection for everyone to use. Of course, RV's can run lights, etc., on the internal batteries, just let the RV run a couple of times each day. Generators are needed to run microwaves, AC.
As far as how MANY showers, that depends (like I said) on the water tank and how you take the shower. I typically do not leave the water running during the whole shower, soak down, thurn it off, soap up, rinse. It's amazing how little water a shower takes.
There will be a dumping/cleaning charge on return, it cost me (no option) $150 for a Class A 34 footer.
Like anything else, ask them point blank what you could possibly have to pay on return. Ask about hidden charges. Usually, everything is spelled-out in the rental agreement. Read it!!!
Also, ask if there is a discount for early reservation, early payment, etc. I've already done our deal, saved 9% by paying in advance.
Hope that helps. Hopefully Jay (Offroadr) will add to this topic, he is MR. RV (owns his rig).