The answer depends upon several things. Does the Tioga have a compartment that is designed and wired for installing a genset? If so, do you then plan to install this generator in it? If so you will need one of the gensets that were designed for RV use and come from people like Onan or Generac. They can be purchased from any RV dealer or supply store.
If you just want a genset to carry with you and to plug into for power then you could get any of many different brands but you want to make sure that it has voltage regulation as most of the lower priced ones do not. It might be wise to also consider how much noise it makes if you wish to use it in areas where others are camped as a noisy genset does not make for good neighbors.
As to the size you need, that too depends upon you. If you want to be able to operate your air conditioner, microwave and anything else in the RV all at the same time, you probably need at least 3.5KW. If you can live with no air conditioner, probably 1.5KW is plenty.