I think the relative "stealability" of each type of RV is a nonissue. I don't believe RV's get stolen very often (as compared to autos & trucks) and I would be surprized to find that of those that are, no one type of RV has a much higher theft rate than another.
The eternal question of which type of RV is best has no answer. It's so personal and so riddled with tradeoffs, that most often anyone who has an opinion (and all of us do) only got there by a lot of trial and error. Since 1973 I have had truck campers, chassis mounts, ball trailers, fifth wheelers, and am now on my second motorhome, pretty much in that order. Each, for me, had shortcomings that led me to the next, and each had strengths that I gave up when I changed. For instance, we quickly outgrew the space in the truck camper (in fact, if there has been one central "theme" that has held out over the years, it's been space - more and more space!) but even today, I sometimes miss the versitlity of a good four wheel drive truck and camper. On a "dollar per square foot" basis, trailers are hard to beat, even factoring in the cost of the tow vehicle. I moved from a ball trailer to a fifth wheeler because, to me, the 'towability" and stability is so much better. I moved to a motorhome from trailers for the reasons already stated by others above, of convenience and security. I feel much more secure in a motorhome in rest stops, etc. Doing your homework is critical. but the ultimate answer, I believe comes only with experience and an understanding spouse and banker!
Best of luck!