I camped by myself for about 2 weeks on 100 gallons of fresh water. I did have a place to drain my 42 gallon grey tank, and the 59 gallon black tank never got full - even in a month.
Yes I think you can go a week or so - perhaps 2 weeks without dumping. I like the idea of dumping your tanks every time they are full, and you will get an idea of how many times it takes to fill the tank. Then call the guy to dump your 300 gallon tank, and dump the black tank and RV tank all at once, getting about 375 gallons all at once.
The charge is more for the labor to drive out there, than the actual cost per gallon to dump it. I think they even charge $20 per portable toilet serviced weekly.
You can also get thrifty with your water too. If you can recover some of your shower or dish water, you can dump it outside, and also save some in a used gallon container to use for flushing the toilet.
Fred.