I have seen rental RVs parked along Haigeng Daba. Next time I see one I will photo the company logo and send to you. I thing one had Chongqing number plates. You could maybe take the train to Chongqing to collect.
I have also seen RVs parked by the lake management building at the end of Haigeng Park on the corner of Xingti Lu and Hubin Lu. I am not sure if this is sales or rental or both.

We did something like this with our students a couple of years ago. I was part of an education program attached to a school. We were later informed by the school authorities that it is illegal to take school students on overnight trips. You might want to check this out.
Having said that, there is a lot of field-craft you can do in a long day. Even erecting a tent can provide an engineering type challenge.

There was a pool table in O'Reilly's pub on Beichen walking street. But I am not sure of opening hours.

JD is probably safest. Some of the things that you buy cheaper may not be fully functional and have faulty cells. Like LED floodlights the cheap ones often have faulty cells, but you would never know unless you know what to look for, because things that fail QC are just offloaded at the big markets and then trickle down. They usually trickle down to the small shops and small suppliers. Buying from flagship stores via JD or TMall is something I would recommend.

There are loads on Taobao. s.taobao.com/[...] Prices vary a lot, and so do the power outputs. There are many folding portable sets, but these are pretty large when folded out. You need to find out the Wattage you will require to charge your tablet, I am sure there is a Google answer somewhere out there.
I would also do a search of JD.com, the consumer feedback might also be a bit more useful on JD.