Recommendations for affordable hotels or holiday parks on the south coast

[Blueberryme]

I am looking for some recommendations for nice but affordable hotels/holiday parks/B&B around the southern coast of England please - more around Dorset, Brighton, Weymouth, Bournemouth etc.

We usually go to Premier Inns as we prefer to spend money on activities and experiences but I am looking to book 2-3 nights away a few months after DH has finished chemo and when he feels up to it, and these are areas we would like to see more of.

I’ve seen some nice looking adverts from haven. You could also get a great deal on air bnb if you book midweek term time

We've just been to Rockley Park caravan park in Poole, been there about 5 times. Lovely site, nice restaurants and chippy, good choice in the shop on site. Very clean and as we went out of season it was relatively quiet. (Although they were practicing helicopter take off and landing at the nearby naval base!) That was mostly in the evening.
If your dh doesn't want to drive, there's a bus that goes into Poole, possibly Bournemouth, didn't look, that goes every hour at 10 past the hour.
I've been to several Haven sites, this one is my favourite but the one at Devon Cliffs comes a close second.
Don't go to Watchet, nice site but bugger all around there unless you're prepared to drive quite a way.

There’s a brand new Premier Inn, which opened last Thursday in Bognor Regis, right on the seafront. You can easily get to Littlehampton, Chichester and Brighton on public transport, or by car.

My (adult) daughter loves going to the Devon Cliffs holiday camp near Exmouth. A bonus is that it is right next to the World of Country Life which is fabulous for children, and has hawk flying and shows, a little land train through the deer park (you can feed them), lots of animals and a vintage vehicle section.

It is a bit further away than you were thinking though. Exmouth itself has a nearly new Premier Inn right on the seafront and the beach is wonderful. Great boat trips either up the river Exe or along the coast as well, including wildlife trips (seals, sometimes) and bird watching.

We've stayed at Wareham Forest Lodge Retreat a few times - reasonably priced but also location good for Arne, Poole, Corfe Castle, Studland. Not much on the site other than a shop but is very close to everything you need.
Also Green Hill Farm Holiday park in Landford - in the New Forest so a bit outside of what you were looking for but it is such a lovely relaxing location and the lodges are really nice. Near to Salisbury, so nice for a day there.