If you take a look at my pictures on Webshots you will see that I did exactly what you want to do. The left tap brings water from the on board tank via a Surflo diaphragm pump and the right tap supplies from the city water hook up. You must use a pressure reducer on the city water. They are cheap and just screw onto your hose.
If you use a diaphragm pump it can be mounted any where and does not need to be primed where the other types need to be mounted lower then the tank outlet to keep the prime. That can be a problem to find a spot to mount it. The diaphragm pumps are a little more expensive but in my opinion are worth it. It is also a good idea to mount a small red LED or a flip-flop switch with a lite in the pump circuit to let you know when you have left the pump power switch on. I have heard stories of people who have left the pump on when they are on the road and the vibration has opened the tap enough so that the pump came on and filled the sink to over flowing and flooded all over the floor.
I have disassembled some of those hand pumps used in RV's and I must say I have never seen such poor quality and bad design in years. But I am finding that with a lot of the RV products.
To go to this type of system, you of coarse need to have an on board 12v power supply which is a good idea as your trailer can be stand alone without having to be hooked up to your tow vehicle. There are several threads here regarding those type of setups.
In addition to adding the tap set, pump and battery
, I also added a In-Sink-Erator Instant Hot Water dispenser. It works great for washing dishes, making a quick coffee etc. when you are hooked up to 100v shore power. It produces approx. 1 1/2 imp.gals. of hot water per hour. The unit I installed was their smallest one. There was just enough room between the fridge
, sink and furnace
for the under counter tank.
So far I have been very pleased with the results and have encountered no major problems with this setup. If you do a search of the threads there are numerous other options that members have come up with that work well also.