Rhine/Romantic Road in Feb?

Here's a few things to keep in mind. Weather is not your friend for rural site-seeing at this time of year. Not because of the cold, which is manageable, but because of the overall damp air. Meaning, visibility is low and everything looks kind of gray. This has less effect in the towns themselves, but it means that scenic driving trips aren't so scenic.

"would be completely desolate this time of year and if we would have a hard time finding things to do?" Yes and no. I've found that some restaurants along the Rhine close seasonally, but that's about all. Things like the castles tend to stay open, although they may operate at reduced hours. Outside of the Mittelrhein, I don't see much difference in towns otherwise, except fewer visitors and no festivals.

Finally, realize something about the Romantic Road. It's one of about a hundred Ferienstrassen that criss-cross Germany. Although the towns along it are certainly attractive, you can find towns like the all over Germany (well, perhaps not so much in the lower Rhine-Ruhr area). If you have a specific interest in one or more of the towns, by all means, check them out. But the road itself is not the attraction. Personally, I find the Fachwerkstrasse more interesting.

There are some good destinations on the Romantic Road. But there's no need to follow the route exactly since Bavaria is full of interesting places to visit. In the smaller towns, you will find that when the sun goes down early and it's frozen outside, there is no evening strolling to be done, and there are precious few establishments to tuck into. I would find a room within the larger towns and cities, generally, and visit some of the smaller ones during the daylight hours if they have stuff that interests you. I would also be reluctant to plan a car trip in winter because of ice, snow, the obligatory winter tires (at your expense,) etc. Most towns worth seeing are well enough served by train so that getting around is easy as long as you aren't married to the idea of pursuing every last km of the Romantic Road.

Würzburg is a train hub and an ideal Romantic Road city to use as a base. You can daytrip to RR towns Rothenburg and Weikersheim, but also to other non-RR gems like Bamberg, Nuremberg, Iphofen, Marktbreit, and Ochsenfurt.

Iphofen photos

Ochsenfurt and Marktbreit video

Bamberg photo

Train travel is dependable and inexpensive with the Bayern ticket:

Bayern Ticket details

(cont.)
Visiting the Middle Rhine Villages is possible in February. Koblenz or Mainz are decent-sized cities you can use as a base town for daytrips by train; find a hotel near the station. I much prefer Mainz, but Koblenz is closer to certain sights and towns, particularly Braubach (Marksburg Castle, open year-round for tours, very good) and Linz am Rhein north of Koblenz (maybe the most scenic Rhine village of all.)

Trains run from Mainz and Koblenz all day long along both sides of the river; I'd mostly give Bingen and Koblenz a skip but make stops in Linz am Rhein, Boppard, St. Goar, Bacharach, and Oberwesel (or whatever towns you have time for.)

If you stay in Koblenz you can use the 3-day VRM mini-group ticket (42.40€ covers the 2 of you for 3 days) to all the above destinations except Bacharach (which is just a few km outside the zone of validity, so buy an add-on ticket for that trip or include a stop there on your way to or from the area.)

VRM mini-group and other daypasses

If you stay in Mainz, use the Rheinland-Pfalz ticket daypass (26€/day for two, covers all destinations above) for daytrips along the Rhine:

R-P ticket