I've used the 25-30 gal tanks in the bed of my truck. I currently have a 35 gallon corner tank in my midtack 

I find that with 4 horses, 35 gallon is one day of water. So if I  know that I will be dry camping and need more than the mid tack tank, I will throw the tank in the truck bed and fill it. I've thought of buying another 30-40 gallon tank for the hay rack.  I kinda like the hay rack tanks, because they have plenty of pressure from the gravity. The tank in my mid tack , is  a little ackward to get to, the outlet is close to the floor. So I don't get near the pressure as I do from a roof mounted tank. Especially if I want to rinse off a sweaty horse. HWbar suggestion of an electric pump sounds great. I just haven't found the need yet.

I personally use the water out of the tanks for the horses and to clean up with. Wash dishes, wash hands etc. Any water I want to drink I bring as bottled water. I always fill the tanks with potable water, but it's difficult to completely empty the tanks. So I worry about any thing that might grow in the tanks between uses.

We have a neat 40 gallon tank in the mangers of our trailer with a small pump and hose to reach the water tub or even reach the main water tank on the trailer incase the LQ runs low.  Our trailer is 8' wide and this tank is mounted between the inside wall and wheelwell and fits flush with the top of the wheelwell, so we lost very little storage.

When coupled to an external supply, most RV water systems depend on the city water supply, for the pressure to operate the LQ appliances. Their built in water pumps are usually not plumbed to provide the necessary function or pressure, when the water is sourced from anything other than the built in tanks.

Gravity feed only provides one pound of pressure for about every couple feet of height. Many RV systems commonly use pressures up to 60 PSI. To have an equal head pressure with a gravity supply, would take a tank placed at a 139 ft. height.

I'm not sure by your description, if in fact, your LQ has a water pump or water tank installed. If not, a RV pressure pump could be added to your external tank to provide the necessary system pressure. If it does, a lower priced displacement pump, could be used to fill your present internal LQ tank from a truck mounted tank. You also may be able to gravity feed your LQ tank without a pump, if its maximum height is entirely below the truck's bed height.

If you have an internal pump and tank, the city water inlet line could also be re plumbed to directly feed the inlet line of your system water pump. This would enable you to gravity feed the city water port, and use the LQ pump for pressure. This would by pass your internal LQ water tank as a source, when two way valves are used. A couple of valves could be installed to redirect the external water source, depending on if a gravity tank or city supplied pressure was being used.

There are various ways to supply water for usage in a LQ. You have to first determine what equipment is presently installed in your LQ, before you can then decide on using alternative water supplies.

Gard

No water storage tank...no water pump. Only a city water connection and a hot water tank. I usually only camp with hookups. But there are some places I would like to go that I would need to primitive camp. So would I need a pump to pump the water from the barrel to the water connection to have water pressure?

I also plan on buying a generator next year to solve the total electric issue. But I have to have water especially horse water.

If you are concerned about sufficient horse water and if you have a hayrack, the 48 gallon half moon works well.  Like Painted Horse said, with that much height, there is enough pressure to get decent flow.

"If not, a RV pressure pump could be added to your external tank to provide the necessary system pressure."