In reply to Rog Wilko:
> 1. When's a good time to go for weather etc.?
Geography is important here. The Coast Mountains in Canada and the Cascades in Washington are part of the same E/W watershed - wet side is west (temperate rain-forest), dry side is east (semi-desert). Visiting wet side crags before July / after September is risky weather wise, you could lose half your time. The east side is climbable all year round, but can get hot in summer. For my money, the best crags are on the wet side.
> 2. Do tourists crossing the border from Canada get decent treatment?
Assuming you have a UK passport, UK is on the visa waiver list, therefore all you need to drive into the US is an I-94, which you can get at the border and costs a few quid. Its valid for 3 months, but if you don't ever hand it back, they don't like because they don't know you've left and they could stop you re-entering on some future date. You can't post it, but you can usually hand it in at an airport. To fly in to the US, you will of course need an ESTA.
> 3. Where might we climb?
Within reach of Vancouver (with minimal approach) with driving distances from there:
Squamish Area - Canadian wet side - 1 hr north. Yes, its granite, but there's lots of cracks, single pitch to all day. Not much below VS though. 'Squamish Select', Marc Bourdon (2012).
Skaha - Canadian dry side - 4 hours east. Gneiss trad and sport, soft grades, good vibe. 'Skaha Rockclimbs', Howie Richardson (2012).
Leavenworth - Washington dry side - 4.5 hours southeast. Similar to Skaha, but better. Another hour east is Vantage, popular sport venue, never been. 'Leavenworth Rock', Viktor Kramar.
Washington Pass - Washington dry side - 4 hours southeast. Easy access alpine, not been yet.
Washington wet side has lots also within easy reach of Vancouver, mostly minor, but notably Index (4 hours southeast) is amazing, but very limited below HVS, and its Granite. Falcon Guide - 'Washington'.
Smith from Vancouver is about 7-8 hours I think. Smith is great, better for Sport than Trad, on the dry side but hotter than Skaha or Leavenworth in Summer (its in a kind of sheltered bowl). Falcon guide 'Smith Rock State Park'.
> 4. Anyone had experience of hiring a motorhome over there?
No. But it will surely be much cheaper in the US, most things are, food & petrol significantly so.