Yes, stick a hose into the lake and screw it to the input port of a pump. Screw a hose between the output of the pump and the fresh water tank input on the RV. Pump until the tank is filled to the desired level then stop the pump. You may even have an input port to the RV pump built in (for winterizing the system).
Have the ambulance service on speed dial, for when you come down with the awful awfuls.
Some kind of mesh filter on the input hose in the lake will reduce the fouling of the pump by chunks of stuff in the lake. A good inline filter should cut down on particulates and some contaminates, but may stress the pump too much. Make sure you you don't drink the water unpurified either by a true 'purifier' filter or by distillation or at the very least, boiling or purification chemicals. And make sure you drain and sanitize the water system before using the RV at a place with potable water.
Personally, I would not put lake water into my RV. I would purify it into an external container, then pump that into my RV.
Welcome to the forum. You might explain in more detail why you would want to pump water into the rv from a lake. Guess if maybe you are in Alaska the lake might be pure enough but I doubt it with todays pollutions.
I probably should have explained a bit more about why I want to take water from a lake.
We recently bought a lot to build a year round home in northern Canada. I would like to put a travel trailer on it for about 3-4 yrs while we build. We would use the trailer about 3-4 weeks per year during the summer months.
I needed the water for the washroom, not sure exactly how to go about it.
I have also considered buying a porta potty and some type of portable shower what do you think.
PS most people who have built on the lake so far are using lake water through some type of filter, not sure if I would do that or drill a well
All of those who I know that have done what you propose have also either hauled their water from town, or they have a well drilled. If what you want is to purify the water, all you need is to put in a water treatment system like cities use but on a smaller scale. I think a well would cost less and you will need something when you build anyway. If you somehow pump lake water into the RV water system, you could contaminate the entire RV system, you may get foreign materials into the system that cause problems for it and you may in time find that it was not a good choice.