Well, if they are ready to buy now, then go for it. If you wait a year or two, something else will come up (like needing money for food, lol!).
What make and model truck does the son in law have? At first thought, a diesel half ton should be fine towing 7,000lb fully loaded.
What is the ratings for the truck?
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2021 F150 2.7
2004 21' Forest River Surveyor