Baltimore/washington Parkway with a 5th wheel tralier

There is really no advantage to taking 295 vs 95. It is only 2 lanes each direction and a bit more congested than 95.
While I don't believe you'll have any clearance issues, the road is not set up for trucks. It is a commuter road designed with lots of cars in mind.
I understand 295 allows for more direct route however the traffic patterns eliminate any advantage. Take 95 to 695 south to the Key Bridge.
In the wee morning hours or late evening hours(non rush hour times)you may take 695 north around to 95 N as well.
295 may look better on paper however it is a Parkway with lots of commuter traffic.

We've done it in the 5'er and Endura with no issue but I agree with ependydad-the topside is less stressful.

The lower half of the BW parkway from I495 to MD198 has older stone underpasses for the hiway traffic. But I cannot remember the official height. Being one that grew up on Long Island, I can tell you I have thing against driving anything taller then my F350 on any road named a parkway unless I have been around it and know otherwise.

I researched this recently:
http://www.learntorv.com/2013/11/road-tunnel-restrictions-near-maryland.html

When I contacted the National Park Service, they had this to say about their "commercial truck restrictions":

Is this vehicle registered Commercial? And YES we do have height restrictions. We have bridges under passes, which would not allow anything over 13 ft.

I asked where the 13' overpasses were but didn't get any response.

IMO, stick to I-95 instead of 295 (Baltimore/Wasington Parkway).

There are no height restrictions with the Francis Scott Key Bridge. You have to take it or the topside of I-695 around Baltimore since you can't go through the Baltimore tunnels with propane. I prefer it over the top side.