Tiffin has a good rep for customer service and well-designed coaches, whereas Thor Motorcoach is known more as a "value" producer. However, Thor Industries also owns some other major brands as subsidiaries, including high end Entegra and Tiffin, so don't get confused between Thor Motorcoach brand and other Thor-owned companies.
I'd add Newmar & Winnebago to your potential purchase list - both build darn good coaches, whether gas or diesel. And while I'm not a big Forest River fan in general, I'd look at the upper end of the Georgetown line too.
Rev Group's Holiday & Bounder lines of gas coaches don't seem to be the good values they used to be in before 2009, but that's based on a quick walkthrough rather than actual experience.
I encourage you to look at the high end of each manufacturers line-up and go old enough to stay in your budget range. The entry-level brands tend to have a lot of short cuts taken under the covers where newbie buyers won't notice until they've had them awhile. They skimp on the chassis, the wiring & plumbing, use cheaper materials, etc. Whereas the age of a used motorhome is almost irrelevant since most are lightly used and low mileage compared to cars. Condition is far more important than model-year of the motorhome.
