Any auto-parts or RV would have standard blade 7 pole receptacle, but noted you have a Lance-using the proprietary plug?
If adapted to truck bumper plug I guess you could use what you have and use a standard receptacle installed in bed, and your adapter to it, or change camper to standard 7 pole.
Probably better to order Lance bed plug to install though. Advantage of using the Lance is you could run 8ga for the charge & ground, now or later. Otherwise just convert to standard. Most 7 poles will only accept 10ga.
As to Wednesday, I dont see any reason to not go a head and line bed. The new plug shouldn't be 'sealed' to bed and replaceable, no reason that you cant add after its lined.
Personally I'd rather have it lined under the receptacle.

I added our aux (Top) plug (Pollak or Hopkins), after lining. I used some zinc primer to paint raw edges after cutting hole. I pulled the dealer install (Bargman) one loose to get behind it when lining.