I would consider building a roof over the trailer so you don't have to worry so much about the standard RV leak points.
I don't think stationary trailers blow over very often, I think the tie down idea is more for stability in the wind.
If there are a bunch of other trailers around the lake you might just wander over and strike up a conversation or two. You will probably learn quite a bit from the people who have been there for a while.
Don't put the trailer where you think you might want to build a cabin. That way you won't have to move the trailer later to start construction.
I know nothing about septic, but you will probably want to find out if it is even allowed on your lot. I'm imagining there must be some sort of regulation about the distance from the lake shore.
If you build septic, permit and build it for the future cabin. Then just run a sewer cleanout or two to the areas where you might park a trailer or trailers.
If you run into an ordinance that the trailer must be mobile, find out if it's good enough to block it up but leave the wheels on it. then just skirt around the trailer to hide the wheels.