In the end, a 25' unit can be towed with a 1/2 ton. A 3/4 ton would be better, but it can be done. I haven't looked at the 1/2 ton ratings in a while, but say if they were about 7500lbs I'd feel OK with it. Of course as Pieman states a diesel has more power (and more cost), but it does get better fuel econ. To be honest, few gassers will get awesome gas mileage towing be it a 5.4L, 5.7L, 6.0L, 7.4L or 8.1L. I've read and found folks getting between 10 and 12mpg. One time I got 12.9mpg towing the Safari with our 6.0L.
Of course, if you're towing a 19' Bambi like the thread starter does, a 1/2 would do the trick without question. IMHO, once you start to get north of 22', you start to fade out of 1/2 ton and start to fade into 3/4 ton. Depending on vintage, what you carry, options, number of passengers, etc, the 25' begins to see areas where a 1/2 might and a 3/4 will.
Personally, I don't think I'd tow a 31' coach with a 1/2 ton, that's just me, I'm sure others would agree and disagree. The cost of new trucks are about $10,000 off where it was a bit over a year ago. There has never been a better time to look at tow vehicles (from the big three), that goes for diesels too. But if it's not in the budget, it's just not in the budget.... but it is a safer bet once you get into the larger coaches.