I'll put a vote in for a stealth B. How often will you realistically be taking grandkids along vs touring yourselves? Do you plan on touring along the crowded/18' parking US Eastern coastline much? We all have our priorities, but I rather deal with tight house for 1-2 weeks vs storing/driving a big one for 50-51 weeks.
I've slept 4 (3 beds, 1 air mattress/sleeping bag in the aisle), and had seated dinner for 8 (rain @ tailgate party) in my RT170P before. Sure it's tight.. certainly wouldn't want to do all the time, but a week or two a year is doable and can be fun for kids in a submarine logistics/efficiency kinda way. A tent outside can also be fun for the kids to sleep in and a video gaming machine plugged into your TV can occupy them for hours - silent and unmoving in single back seat. Even my smallest B can be split into two living/dining/sleeping areas using the shower curtain.
I love using my 170 like a second car, especially for the day trips and taking our dog anywhere. For me, a C class would sit in storage most of the time only use for multi-day trips. Early 60s is still young... go for agility!
(Then again, I never grew out that childhood fascination with compact/folding/versatile/multi-purpose things and am keeping my eye on the Tiny House movement.)