STP is as close to dead flat as a ride comes and I assume RSVP is also a pancake. "Back in the day" when I did double centuries in eight hours, my wife could hold my wheel all day long as long as the terrain was flat even though she couldn't ride over 20 mph on her own. If your drafting technique/spacing is up to snuff, you should be able to stay with almost anyone on a flattish ride. Is your husband a "good wheel"? Maybe both of you could use some time on the rollers in order for you to better hang on to him. Maybe he is too trim to offer a good draft. Nothing like a baggy T-shirt to make him a better leader.

Of course, my preferred solution to this all-too-common problem is to get a tandem. With a tandem, the weaker rider never prevents the stronger one from putting out all the effort he/she wants to.