Hi,
Somehow the people asking questions seem to expect the people answering the questions to know as much about the product they are looking at as the person asking the question.
So how heavy is the RV?
Do you plan on driving in a lot of mountains, or just back and forth near the home or places like Florida?
Do you plan on towing a car? Probably not required with such a short vehicle, you can park it in most parking lots.
The 5.4L makes upwards of 300 HP, not that much less than the V10 engine. They in fact share the same piston size, and stroke length, only change is number of cylinders.
If the change to 5.4L means changing to the lighter weight capacity E-350 chassis, I would not make that change, even if it saves $5,000. I would want to keep the E-450 chassis, and get the smaller engine if you don't plan on using it in the mountain or towing much. Yes you can still tow fine with the 5.4L, just not as much. I think the difference is you can tow up to 18,000 GCVWR with the 5.4 and 22,000 with the 6.8L engine. So 4,000 more with the larger engine.
So if the motorhome is say 11,000 pounds, then the lower capacity chassis might work (E-350) but if heavier, then the 14,050 pound E-450 chassis is desired, in case you want to carry extra weight sometime. The E-450 chassis is less likely to "Wallow" around, as it will have stiffer springs, and keep upright better.
Fred.