I remember when you first posted trying to remember if I had ever been on NH 118. I think I may have once in the dead of winter, and I remember some roads on that trip were definitely rather non-RV friendly and rough, but it was (a) the dead of winter and (b) on several roads, some of them very much back roads.
100 to 103 should be fine, though there are some curves and hills, which are pretty much unavoidable in this part of the country. 73 I'm not really familiar with; on google street view it does look a little steeper and curvier than the others, but to my estimation not too bad. I'd take my class C over it without hesitation (but with some care).
US7 South to US4 would be a reasonable route as well, and probably not any slower in practice (assuming you're heading south on I-89; if north, sticking on 100 would be decent and put you through some pretty countryside and a very pretty leg-stretch stop at Moss Glen Falls in Granville).
