We bought a used pop up when we first started our RVing adventure. The odd part was we looked at three (at private seller's homes) but only one person had it hooked up to water and electric so we could use the A/C, sink, etc. After a pretty thorough inspection and a bit of negotiation we bought that one. When it came time to sell it a year and a half later we had it hooked up so we could show potential buyers how things worked. We sold it for what we paid for it to the second person who saw it. The first one called back a few hours too late and made an offer.
I guess the moral to my story is use everything
as well as visually inspect the pop up.
Good luck and happy shopping! I hope y'all enjoy your pop up.