How often and how long you dry camp is a huge fctor in all of this.
A built in Onan is without a doubt the most convenient way to go.
LP installation is the siplest way to use a built in.
If you want the efficiency of gasoline, than you need to provide a gas tank which adds to the complexity.
Portables are fine but will require transport and handling as well as gas can etc.. Portables are not as user friendly vs. the push button convenience of a built in
If genset is in truck, than truck will need to stay with RV in order to have power. If truck leaves electricity leaves. With that in mind a portable on a rear rack is doable, but it requires a more complex installation.
THere is no simple answer, the correct method is a direct reflection on how you intend to use it.
A final decision/method should not be made without seriously investigaing a large solar package.