Schomer is correct, a low-end unit may be great for occasional use camping, but if you're going to be using it as much as it looks like you will (out of town work) for days at time, you'll wear a cheap one out faster, and be spending a lot of time fixing stuff that falls apart.
I have a medium unit, a 30' Salem by Forest River, and it's great for camping but would be somewhat miserable for more than 2 weeks at a time.
If price is a concern, a high-end unit like Holiday Rambler or Carriage can be bought used, and will give you more value for the money.
Use your eyes, look at the fit & finish, the trim pieces. If it LOOKS slapped together, it usually is !!!!
Get a 5'er if you can, they have more room, and pull much, much better.