For towing a trailer, you need sufficient rear axle capacity to support the tongue weight of the trailer plus the RV loaded for travel. That really depends on the trailer and the RV in question. (Keep in mind that the long rear overhang of most class C's means that the force of the tongue weight will be multiplied somewhat on the rear axle due to the motorhome acting as a lever. That's assuming you don't have a weight distributing hitch, which is a pretty safe assumption.) Practically speaking, that generally will imply a dual rear wheel setup, but the reverse is not necessarily true; not all class C's with dallies can safely tow an enclosed trailer with a couple of motorcycles (which is what, maybe 2000 or so pounds total?). Also be sure to check the tongue weight ratings of the hitch system; sometimes it's relatively limited.
The overcab sleeping area can be reasonably comfortable, but it does have the limitations you describe: less headroom, the need to clamber up and down, and for two people the need for one to crawl over the other should they go up or down in the wrong order (or need to make a bathroom run in the night). I don't find mine particularly obnoxious to use, at least for one person, though I'm a little younger than you are.
The rear bed is more convenient and is, or at least can be, more comfortable: there's more space for a nice seamless mattress. It's much easier to make, particularly nice if you don't care to use a sleeping bag.
Do you have particular reasons to especially want a short RV? They do have some advantages, which at times can be very important--such as if you have a garage or parking pad that is only so long. However, some of the things that people think would be advantages are really not much different, such as the ease of driving one down the road or getting noticeably better fuel mileage.