I look at motorhomes (and trailers too) like cars. You lose the most value off the top if you buy brand new and go to sell within 5 years or so.
BUT, like with cars, even when buying used, you're still going to lose money on it. Very rarely do you buy a vehicle and then are able to turn around a few years down the road and sell it for more than you paid for it (which is the intention when you buy a sticks and bricks house).
In the end, I don't think anybody should look at a recreational vehicle as an "investment" in the typical financial sense, no matter what you're using it for...whether that be for weekends here and there, extended travel, or full timing. Few people buy them because it will give them a return on their money. You buy one because you want to enjoy it and the type of lifestyle/travel it can provide. Now, if you look at it as an investment in your quality of life, then that's different :)