Have you thought about how frequently you'll want to be taking long trips with 5 people?
I love the West and enjoy trips there. The driving is very different than in other parts of the US due to the vast distances. 5 people in a small truck would sure get uncomfortable (it would seem to me) after awhile. I see your concern about doing that trip with your current equipment.
You seem to like your current truck very much and it is probably well suited for your lifestyle. So if you're not planning many long trips with 5 people maybe renting a "chase" car is a good idea. That is double driving though. Cost of gas/rent would be much less than changing to a larger truck. If your trip out West is a one timer- maybe the chase car is not a bad idea.
However, if you are seeing yourself taking many trips, especially long distance trips, using a truck to pull a 5th wheel, then looking at converting to a larger more comfortable truck for 5 people might be worth considering. That will be an expensive option, it also means you'll be dealing with a larger truck for around-town times.
Driving around out west is a wonderful experience. Taking your 5th wheel up (and down) mountains will require a strong vehicle. I suspect your current truck is capable (?) but if you convert to a larger truck, you'll want to be getting sufficient power so your driving experience is pleasurable. Once you spend time out west experiencing all that it has to offer, the chances are that you'll want to return again and again.
Good luck with your thoughts. Lets see what other thoughts forum members have to contribute too!