When my late Mother in Law was traveling with us at age 93 I installed one assist bar in the stair area and one next to the toilet in the bathroom.
I went clear through the wall with round head machine screws and fender washers on the "blind" side. Acorn nuts on the handle side. One of my blind sides was in a closet and the other in the washer/dryer cabinet which we use for bathroom supply storage.
On a previous trailer the one next to the stairs came out not so "blind" on a short wall next to the basin. I took a piece of clear grain 1/2 by 3 trim and routed a decorative edge all the way around. Painted the board and bolt heads/washers to match other woodwork in the bathroom and just left it in plain sight. Nobody ever mentioned it.
The grab bars are available at Lowes and the tubes can be cut to any specific length to fit your space.
Formerly posting as "littleblackdog"
Martha, Allen, & Blackjack
2006 Chevy 3500 D/A LB SRW, RVND 7710
Previously: 2008 Titanium 30E35SA. Currently no trailer due to age & mobility problems. Very sad!
"Real Jeeps have round headlights"