Yall obviously probably know several of your wife's parents' neighbors at the lake.
I am sure the neighbors know and like you.
Perhaps, ask one if during "riding season" if it wouldn't be asking too much, "if we could occasionally, leave the old Toyota pickup(Jeep, FORD whatever..) in the edge of the woods to the side of your driveway near where you park your boat trailers, the idea being to be able to make this scenic ride to --or-- from the lake when the weather permits........reason is to be able to either Finish the 50 mile ride from my home to your place at the lake....OR......to drive here, leave the vehicle....and ride the 50 mile ride from (here) your lake house back to my house....
If you know them well and if they think highly of you, and if they have space to leave a car parked that won't impact space requirements if they were to have a party with 15 or 20 guests or something like that......
It's not like you're Spicoli, the dead-head, 21 year old boyfriend of their grand-daughter that wants to park the 1983 Dodge Motorhome there and run an 110v extension cord for electrical power and live in the driveway with their grand-daughter Julie, at least until Bob Weir & friends, Phish, or Widespread Panic start their tours.
Be kind and considerate and bring them wine and a fruit basket every once in a while, and reciprocate by offering to let them to come down anytime to visit yall's beach house. Also be sure that you and your wife take them out to dinner three times a year to someplace that they enjoy.
Treat them at least as well a you'd like to be treated , if as a favor, you were to let someone, park a vehicle at your lake house.
Remember that nobody likes someone that overstays their welcome, but if you are cognizant of their needs for parking space for Labor Day, Memorial Day, 4th, Thanksgiving, and Birthdays, so you are ahead of the game and don't need to reminded over and over to please remove your vehicle for this upcoming weekend, or party etc. If you treat them respectfully, like you'd want to be treated, something like that might fly.