Considering that you live at 4,500' elevation (or close to it) you need something that is great in the hills, not something that is 'just powerful enough'.
That said, Banks can do wonders to a 275 HP motor to improve the situation. Call them first, with the engine size, RV weight, GVWR, ect. THey probably have a intake manifold and larger turbocharger that will spool up quicker, and improve your HP rating by 50 - 75 HP! At that point, you would also need to improve the cooling capacity to keep using all that HP. Your radiator is probably sized for a 250 HP engine, and will overheat a little when climbing a long grade, but after 10 minutes at full throttle, you will start back down the other side, so no problem undersizing the radiator a little bit. Change the engine to 325 - 350 HP, and you might find it overheats on long grades!
Your transmission will handle the extra HP no problems. Depending on the mileage, you might want to change the transmission fluids, (it should be done every 25,000 miles or so, but most RV owners never consider it needs changing).
Banks is not to far away, in Azusa CA. I had my Banks system factory installed in my year old 1997 Bounder with a 460" gas engine, and I loved the HP improvements. You will be able to try out the new power on I-15 right away, leaving the factory! They might recommend a larger intercooler, turbo, and intake manifolds. This provides more cool air to the engine, and makes it breath much better. Yet it can be expensive, and if you are getting the 275 HP rig for a great price, it will be well worth it to change it to 325 HP rig, and it will be a great selling point!
Good luck with your purchase!
Fred.
