Adding tanks is probably not going to work. You'll be messing with the weight dynamics of your RV, chances are if they could have fit larger ones in they would have done it.
If you are camping without hook up's and campground showers are an option, make it mandatory. Outside shower works too, the PahaQue Tepee works as an outdoor shower that drains on the ground - depending on your camp site it might be an option. Others have already talked about dishes being done outside, it really is just learning to live with what you have... or upgrading ;)
