A muffler along isn't really going to improve flow all that much....you'll have to step up to a larger diameter piping front to back to notice a solid improvement. With that said Dynomax makes a full line suitable for RVs. Any improvement is a welcome improvement I guess, right? also, K&N fresh air intakes are also something to look into, if you don't already have one.
A-1 RV Mufflers
People will throw funny looks at the thought of aftermarket mufflers and exhaust on motorhomes, because most think those are upgrades you get if you want a louder, more mean sounding exhaust...and why would anyone want that with an RV?? But what they're forgetting is that RVs really benefit from performance boosts. Just look at the Banks power pack (high-flow headers/exhaust/muffler). Many motorhomes come with it already installed, yet if I told my neighbor I was putting headers on my RV, he'd think I had lost it haha.