While I can't give you any advice on storage in the Boston area, I can share with you my indoor storage experience. Simply stated, it will keep your unit looking new. Every time I check it out for a trip, I still marvel at how 'new' it looks. Yes, there is an extra cost associated with indoor storage, (in my case $165/month), but I feel that this is a small price to pay to protect my investment. Should the time ever come when I want to upgrade to a larger unit, it will be just that much easier to pass on to a new owner and I know that I'll get a better selling price because it has been cared for and properly stored, indoors and protected from the elements. In your case, being up North where climates actually change and you have to contend with snow, this will become an even more important factor.
One Long Shot
Beastie - 19'FC
The Beast - Mercedes 350 BlueTec
Held together with the Husky Centerline Hitch