Algonquin Prov. Park is the oldest park and biggest park in the province. Unfortunally that means the sites were design before we had the hugh campers. Also, as its a protected site I would also presume that expansion and updating would be a nightmare in environmental paperwork. It caters to being in the great outdoors, as in backpacking and canoeing trips. I have yet to camp there in my trailer but have done it many times tenting and day trips. It is really a great experience but I can definetly see your point as now that I have a trailer I would also find it harder with the dump and fill stations further than the norm. Algonquin is also so hugh that if you want to use any the hiking trails or museums you need to drive to each one. That being said they are worth the drive. Also the visitors centre is closer to the East Gate. West to East is 56 kms. There are a few gas and convience stores inside the park but $$$$$. Gas Stations and the closest stores outside the park are 31 km (village) to 50 kms (town). Cell phone service is very limited. Do check out the link I posted earlier as it has all kinds of usefull information on Alqonquin.
As for the other Prov. Parks I'm not aware of any having sewage and water right at the site. I'm heading out soon to my fifth one this season. The sewage and water stations are at each park though and so far I have found that I haven't needed to utilize them except for the first and last day. I have an X19H and I do not use the shower at all (Park showers so far have been very clean and convient) and my family of 4 uses the toilet only at night (forces my kids to go out for a walk and enjoy the park).
As for electricity as I said I found some were closer than stated. At Rondeau P.P. it said 15 m (45ft). I went out and bought a 50 ft extension only to find the post was only 15ft away. Thankfully I had it for Bronte P.P. as it stated 20 m (60 ft) and that was correct. This was the fustrating part.
We have alway stayed at Prov. Parks because the sites in most part private.