We did what you planned to do as a section of the long trip we were doing that year. We stayed at Manzanita CG in the park and it was excellent. But has no hookups at all of course. At Lassen in two days you probably won't have time for more than a drive all the way thru the park, stopping at the turnouts along the way. Do whatever Ranger talks/hikes that you can. We were there in late June and the park was still mostly snowed in. Trails were under the snow. The park is at 8000 ft so don't expect it to be clear of snow before mid July.
I think we took 299 across to the coastal Highway and it was fine. We stayed at the Crescent City KOA and visited the Stout and Simpson groves. Both were great. There is a visitor center in Crescent City. The KOA was a standard KOA, fine for us.