If it's not for too long, maybe an hour or more in most cases, the pets would usually be fine without the AC running while you run into a store. It does of course depend on the current weather and outside temperatures and such how long that can be. RVs are better insulated than cars (but not as well insulated as permanent houses).
But, anyhow, yes you can lock the RV and leave the generator and air conditioner running.
For used units, age and mileage within reason are less important considerations than overall condition and how well maintained the motorhome was. A ten year old, 100,000 mile RV could be less trouble than a two year old one with a few thousand miles if the latter was not maintained and the former was.
You might want to consider getting an ex-rental RV. At least the big companies (like CruiseAmerica) do a good thorough job of reconditioning the units before selling them, and they tend to have ones that are built more for general sturdiness than flashiness.
Getting a little bit of comfort and understanding about maintenance and some basic DIY mechanical ability would be a very big help. You don't have to become a professional mechanic by any means, but you'll save a lot of hassle and money if you can wield a caulking gun and screwdriver and tire pressure gauge without trepidation.