Leaving Nor Cal in Early March

We were there late August - early September 2016 and the weather was fine. No bugs, some rain. Cool evenings but daytime 60F-70F.

I have heard some folks say that too early is bug season. May-June?

I'm not sure the Alaska Hwy (97) or the Cassiar Hwy (37) through central BC/Yukon will be passable that early in the year. It was bad enough in late August.
Perhaps others have greater seasonal driving insight?

You are a little early. I would not be too concerned. You may run into some areas. By taking your time you can wait out storms.
Some of the boondocking areas could be difficult to get in and out.
I would be suggesting the Fraser Highway. It is lower in elevation. Yes, it will be raining. Have rig ready for wet roads! The Alaska highway would be my choice for north bound and maybe return via Cassair.
Make sure you carry water in fresh water tanks as hook-ups may not be available. Not sure of your water usage and grey/black tank usage. You may want to figure some stops for dumping every three-four days as that what we do.

I will just talk about the first region you will hit in your way north, BC.
You not only have be watching pot holes and narrow roads, you need to be aware of frost heaves. Those frost heaves if not identified by markers on the side of the road, will snap a leaf spring or send you for a ride to the rhubarb if your not careful.
Also campgrounds in BC historically don’t open until the third weekend in May. Your going to have to find places to dump your black and grey water. You will find campgrounds open Year round, but are they on your route, and if so how far apart are they.
The weather farther north, the colder it will be and you will run into snow anywhere in B.C. on your way north in early March. Heck I’m still snowmobiling until mid April and I only live 3 hrs north of Spokane.
You say you don’t mind boon-docking, those places will have to be in a town and most likely a parking lot. The problem is that as you drive north especially if there is snow on the ground or it is snowing the area that you can pull off and park in, usually are the last thing to get plowed out during winter months. Also you will also be required to have winter tires or Mud and Snow ( M/S ) tires on 95% of the hwys in BC until March 31st.
Don’t get me wrong, you can do exactly what you are planing, but you should put some thought and a lot of research in to leaving that early heading to Alaska. You say you have some cold weather camping below zero, well in my opinion that’s not winter camping. I have done some winter camping in my camper while snowmobiling over the years, but that’s only for a short period of time and I don’t use any of the campers plumbing.
You also say you don't have any winter driving experience, and pulling a 5th Wheel on winter roads is not fun, and if you have never driven in snow, and not pulled a heavy trailer you have no idea of what you can expect or can happen.
I have only touched on a couple of things you need to be aware off, there are a lot more.
There are a couple people on the here that monitor this site who do winter camping and live in the Yukon, these two people are the ones you need to be talking with. sue.t & explorenorth
Good luck .

Soup

Returning in August and September should not generally be a problem, though of course the weather varies from year to year. I went to Alaska this past summer and on the return trip was in Whitehorse, YT on September 1st (just to give some rough idea of the timing) and did not have to drive through any snow or wintery weather. I did see a dusting of new snow on a couple of the mountains towards the end of the trip, such as above the Independence Gold Mine historical park not far from Anchorage.

March to May sounds rather on the early side, however. Depending how quickly you go north, you'll almost certainly encounter winter weather conditions. Anchorage itself would generally be somewhat milder than many of the areas you have to drive through to get there, so perhaps it would make more sense to take the ferry on the way North and drive back if you want to take it one direction. (That's not to suggest that March in Anchorage is balmy, of course!)

Towing a 35' fifth wheel would not be my preferred introduction to winter driving, needless to say. I'm not sure I'd really care to do that even now, and I've had a couple decades of winters to practice in cars.

I think DrewE has the best idea. The ferry up the coast sounds like a better option than trying your luck on the Alaska Highway in March. Then drive home when the weather breaks later on.

I'm thinking that's a pretty good idea and maybe would even shorten the trip up thereby giving us more time on the return with the good weather.

What might I expect to pay for a truck and fifth wheel? Best places to embark and dembark? Cheepest? Am I going to be able to find a spot this late? I understand it can be quite full. However we're obviously very flexible and I suppose headed up earlier than most.

buylow12 wrote:
I'm thinking that's a pretty good idea and maybe would even shorten the trip up thereby giving us more time on the return with the good weather.

What might I expect to pay for a truck and fifth wheel? Best places to embark and dembark? Cheepest? Am I going to be able to find a spot this late? I understand it can be quite full. However we're obviously very flexible and I suppose headed up earlier than most.

Not sure.
Try searching for "ferries from lower 48 to Anchorage AK", and see what turns up. Probably cheapest from the Seattle area, but I'm guessing. I believe there are cruise ships that leave from there to go to Anchorage, but I have no idea how far south the west coast ferries go.
If they're booked up, ask about getting on a cancellation/standby list.
Good luck with it.

I just did a rough cost via the Alaskan Ferry web site: leaving around May 1 from Bellingham, WA to Haines, Alaska for 2 adults; 1 infant and your RV would be around $6,000 and probably more because I just guessed on some information to input.

If you drove I'd suggest you save the siteseeing for your return in September. Nothing will be open anyway and you'll then be able to leave later like the end of April if you'd have to be there May 1. Still.... that's very early for your trip. Definitely don't try to cross into Canada in March.

Yowser, that's not going to happen. I could pay that but I just can't justify it. Plus that's nowhere near Anchorage. From Haines that's looking like a lot of driving, which I thought the point of taking the ferry was to avoid that.

I think you're going to have to rethink, based on that estimate and some I did myself. All were very spendy. I used Whittier and Homer as arrivals, from Bellingham.

Best bet might be to sit tight where you are, and keep an eye on weather in central BC, the Yukon, and eastern Alaska, where you would exit/enter. Use Tok, AK as your weather sample as it's the north/south decision point for whether you go to Fairbanks, or Anchorage.
If/when the weather appears to break along the route, make your final departure decision.
The Glenn Highway to Anchorage from there has some spectacular views but it's up/down and desolate in places, and wasn't in great shape in September 2016 when we drove it in a 20' camper van. However, we've driven worse in northern BC and the Yukon.

Good luck with it.