To the OP, I admire your compassion and willingness to assist a caring friend who finds himself alone after years with a companion. The loss alone is a struggle. Your willingness to do something nice is admirable. Karma.
A lot of good and thoughtful replies, particularly from other larger men. Another factor to consider, in addition to his weight, is his HEIGHT. As we've seen from several posters, a man of extended weight but at a height of 6 feet or more, may be more capable of handling the situation than a man who's 5'7" and 400 pounds. Yes, the points made about equipment are still very valid, and are actually probably even more valid for a man with a larger circumference. Shorter and rounder could potentially be more of a problem than a man who's taller and more "spread out".
You mentioned that he was a home bound care taker for 18 years, presumably caring for his wife of 38 years. Surely, this must mean that he has some physical capabilities that allow him to move without too much stress and difficulty, even at a larger size. Perhaps in talking to him about getting about in a trailer (or MH), putting it in the context of his assisting his companion for all those years, may make the conversation a bit easier.
Also, in your conversations, if you're being honest with him, it would certainly be appropriate to mention that certain equipment (toilet/steps/etc) have a weight limit and ask him if he would feel comfortable using those facilities. As previously suggested, perhaps looking for something for him to rent, that is specifically designed for a larger person, may be appropriate.
Best Wishes to you and your friend. He's lucky to have a friend such as you who wants to help, and is honest enough to look at all of the ramifications of helping.