Of course he wants you to BUY a new Sunnybrook.
Tell him with the value of your beloved Sunline, you'll trade him fair and square if he really wants a deal
You sound like my wife how walks around looking at the other campers, some with 5 SLIDES
and comes back with total slide envy (That's what she calls it) I just tell her that we never have to worry about leaking seals, burned out motors or all that extra room to fill with chotchkey.
I'd say after having a slide put in aftermarket, good luck if it starts leaking or anything goes wrong. After going to the Sunline facility and seeing all the work that goes into the trailers to make sure they dont leak, something like that would be a nightmare to put in afterwards.