Might be a sticky situation. Did you receive a title and a Certificate of Registration? Unless he put the unit in storage, there MAY be several years of past registration fees due to the State it resided in. It COULD be even a bigger can of worms if he didn't register it when he initially purchased it and never paid the State sales tax on it. Best bet for "solid" answers it to call the DOT in MA or the State you live in.
Good luck!!! Let us know how you make out!
PS--why not use your insurance agent assuming you have one for your auto, home, renters insurance, etc?
Days camping (2016)----181 days
Days camping (2017) --- 82 days
2016 Wildcat MAXX 28RKX (33' TT), 2007 13' Scamp
2015 Ram Laramie Hemi, w/air suspension
30 years RV'ing
11 different RV's