Hi, Mcwbyu:
I love your sink idea! Clever!
There is a faucet that you can get that will work for both your "on board" water pump or with the land water inlet. So, you won't have to install two faucets. Most RV trailer supply stores have it.
The water problem is that water weighs 9 pounds per gallon. When you consider transporting fresh water and grey water, that adds up fast. That's why so many people carry little or no water and just use the water spigot at the campground. By using a portable jug of some sort for the water, people can be more flexible in selecting their campsite. (They can choose "off grid" sites.) Plus, they can fill the water jug when they get to their site. If they use biodegradable soap, they can empty the dish water near their campsite as they go. This means less travel weight.
With the rising gas prices, we are becoming more concerned with the fact that weight effects gas mileage. But that is just part of the whole picture. Most states have a limit on trailer weight before trailer brakes are mandatory. There is yet another limit before you need a sway bar. In presently smokey California, the state limit before needing brakes is 1,500 lbs. Your tow vehicle may have an allowable tow weight limit as well. When you start adding up all the weights in your proposed build, you'll be amazed at the totals!
When I started planning my build, I was sweating wooden splinters to keep the weight down!
