For the tiny 22 foot Pleasure Way Pursuit, I don't recommend the larger V10 engine, nor the E450 chassis. You'll have far more power than you'll ever need, and the E450 chassis will offer a more harsh ride.
Simply stated, both are
OVER-KILL FOR THE APPLICATION.
If you later decide to tow something, it's not like the V8 won't handle it. It would be equivalent to a massive 32 foot class-C with the V10 towing the same thing. You'll simply take it a little easier.
The V8 engine will yield at least a 10% improvement in fuel economy over the V10. Personally I would opt for a Chevy 3500 chassis if that is an option as it will offer the same fuel economy as the E350-V8, but offer a little more power & cab comfort. But either choice is perfectly matched for that rig.
I don't agree with "More Is Better". It will cost multiple thousands more to purchase and at least 10% more to operate. And your house, it's contents, and occupants will be taking an unnecessary rough-ride beating with every driven moment.
If going with the E350-V8, I would consider the optional 55 gallon fuel tank over the standard 40 gallon tank. It's nice to have more fuel in remote areas where you might want to run the gas generator for extended periods. It utilizes the same fuel from the same main fuel tank.
BTW: The V8 is the same engine as the V10 minus 2 cylinders. They are both Triton series engines with the same dependability.
And that is my opinion.
ADDING:
Here is a Pleaser Way Pursuit

Here is a beast. The Pursuit does not need the same engine or chassis as this thing.

rvit,
If you are concerned with a lack of power with the V8, then test drive both the V8 and V10 before making a final decision. You will learn a lot with a test drive. Being a house on wheels, you would rarely utilize the V10 performance.