Best access to Zion & Bryce with FHU for fiver?

It will be about a 2.5 - 3 hour trip, one way, from Zion to Bryce.
At Zion we stayed at Zion river RV.... at Bryce we were at Ruby's
Both had FHU

I stayed at a KOA mid point between Bryce and Zion. took the toad to sightsee. worked out great.
bumpy

Experience from a few months ago:

I found Kodachrome Basin SP to be an excellent value for visiting Bryce. The FHU commerical campgrounds inside Bryce (or any providing electric for that matter) were closed when I visited in December, but not Kodachrome. Middle of December, temps in the teens, they were open and had nice pull-through sites with water, sewer, electric. Probably harder to get a spot now, but worth a look. Long drive to Zion however.

I stayed at the fed park in Zion, content with just electric. Tried to stay at a FHU just outside the park, campground was part of a Holiday Inn or some other chain hotel. The campground was out of business at the time, despite advertising being open year round (including the sign right outside on the highway) - who knows if they've decided to reopen or not. I think they were maybe converting to a trailer park? Lots of trailers inside looked like they're permanent residents. (Skirting, park-model trailers, etc)

I trust you are aware that the eastern entrance to Zion cannot be used by the larger trailers and motorhomes due to a tunnel. You can get small trailers through, but only by shutting down oncoming traffic and driving down the center of the road. This, of course, costs extra, and as well makes you popular with the tourists.

If you are doing both, I, too, would recommend Kodachrome State Park, BUT it is on the eastern side of Bryce, and not a good choice for visiting Zion.

There are three campgrounds in/near Springdale, two NP, the other private, but you would be best served by getting reservations, as they are popular. There are also a half dozen or more along routes 9 and 89 between Zion and Bryce.