I wouldn't worry too much about mileage unless it's very high. Most motorhomes do not accumulate too many miles, and often low milage implies that it has sat unused and perhaps unmaintained for a long period (which is bad).
Most class C's that you'll be looking at will be built on a Ford chassis. There's nothing wrong with it; it's a solid and reliable design. The Chevy's reportedly get slightly less terrible gas milage and have better legroom in the cab area and may be slightly better ergonomically in general, but both are good.
Do be aware that any correspondence of manufacturer's model numbers to actual lengths is rather arbitrary. Also bear in mind that a 23' overall class C is going to be somewhat smaller inside than a 23' travel trailer. The cab and front end are a good six feet long.
Be on the lookout for water damage, particularly in the cabover section, which is frequently prone to leaks. Also pay attention to the weight and cargo carrying capacities, as some class C's are quite constrained in that area. (Some old ones even came from the factory heavier than the stated GVWR...though that shouldn't be true for any that are of at all recent manufacture.) Finally, take into account the age of the tires; a new set of six will generally run around $1000 or so, roughly speaking. Motorhome tires usually need to be replaced because of age rather than treadwear.
You should be able to find something that's in generally good shape and without serious defects for $30K.