poor flow, Suburban water heater

Confirm it is slow flow on all hot water lines, not just the kitchen sink. (Thinking plugged water filter at kitchen sink as a maybe)

YEP...check valve is internal of the nipple of hot outlet on tank.

They are a plunger/spring/o-ring design held in via a clip.
You can usually remove clip with needle nose pliers and then just pull the rest of assembly out without having to remove nipple.

Then instead of replacing with another cheapo plastic junk just install a shut-off valve in hot outlet line to close when winterizing WH

The check valve is usually installed right at the top connection point on the back of the hot water heater(this is the hot water to faucets line) It facilitates the operation of the single valve hot water bypass.

I gutted one on a winter trip because of what you're experiencing-I literally had now hot water. Just screwed up my winterizing when the trip was over. THese things are generally readily available at RV parts stores.

THE BAD-if you're unit is aging(as in not just a year or two old!) the threads in the tank can be a real pain to start the new check valve in. I'm pretty sure I broke mine off removing it, had to ease the plastic that was left in the thread out and then get the new one to start in crappy aluminum threads. I used Teflon tape but woulda rathered the paste just because I think it woulda made threading it in easier.

BTW-you can do it at home now that you're winterized-honesty, that's what I would do if I was in your shoes-allows a cushion in case you break something else! ;)